feat(host-key): land the HostKeyPolicy seam (OQ-05, policy-blind)

- HostKeyPolicy trait: async, dyn-compatible (Arc<dyn> handle, the
  alkcall injected-handle pattern) — decide(&self, &PresentedHostKey)
  -> bool with host/port/fingerprint as the decision input
- Verdict type pinned to the russh 0.65 check_server_key shape:
  bool is the trust decision; decline reasons stay with the policy
  (declines are dispositions, not errors). Flagged for the wave-1 gate
- Built-in default RejectAll: fail-closed reject-all (russh's default,
  kept); no accept-all option exists in the crate
- async-trait 0.1 added for the dyn-compatible async handle
  (alkcall-precedented; tree-deduped). Flagged for the wave-1 gate
- Tests: default rejects; custom verdict honored; both through the dyn
  handle; the whole presented context reaches the policy

Verified: cargo test (14), cargo test --all-features, cargo clippy
--all-targets -- -D warnings, cargo fmt --check, taskgraph validate

Cargo.lock

@@ -89,6 +89,7 @@ version = "0.1.0"
 dependencies = [
  "alkcall",
  "alkvault",
+ "async-trait",
  "parking_lot",
  "russh",
  "thiserror",

Cargo.toml

@@ -36,6 +36,10 @@ tunnel = []
 sftp = []
 
 [dependencies]
+# async-trait (the alkcall precedent) makes the injected handles
+# dyn-compatible (`Arc<dyn ...>`) — the HostKeyPolicy dial handle needs
+# that, and native AFIT traits are not object-safe at this toolchain.
+async-trait = "0.1"
 # ADR-002: aws-lc-rs only; default features off — no flate2, no rsa
 # (ed25519-only; the tree stays RC-free except the ssh-key pin riding the
 # russh keys path). Never enable russh's `des`/`dsa` features.

src/host_key.rs

@@ -0,0 +1,182 @@
+//! The client-side host-key policy seam (OQ-05 resolution) — the
+//! trust-decision contract the dial surface calls at russh's
+//! `check_server_key` hook.
+//!
+//! Two cases, one seam — the crate is **policy-blind**
+//! (`docs/architecture/auth.md` §Host-key policy):
+//!
+//! 1. **Known beforehand** (trust anchors exist; family-built runners
+//!    mint their host key at assembly time): assembly-supplied pins
+//!    with **hard decline on mismatch**. The crate never hard-codes
+//!    this — a pin policy is a [`HostKeyPolicy`] implementation the
+//!    assembly writes, backed by its own pin store (the host *pin* is
+//!    public config, ADR-005 decision 6; never vault material).
+//! 2. **Unknown beforehand** (vanilla `sshd` on rental fleets; no
+//!    anchor until first contact): prompt-and-record,
+//!    provider-metadata match, learn-and-record, or accept-and-
+//!    continue (the ephemeral-fleet shape) — the OpenSSH
+//!    "add to known_hosts?" moment is assembly policy, implemented in
+//!    its [`HostKeyPolicy`], never crate behavior.
+//!
+//! Crate rules:
+//!
+//! - The built-in default policy is **fail-closed**: [`RejectAll`]
+//!   rejects every presented key — russh's own reject-all default,
+//!   kept. There is **no accept-all built-in crate option**: a
+//!   permissive policy is the assembly's to write when its fleet
+//!   model calls for one.
+//! - Every trust decision is the policy's; the crate never decides
+//!   trust and never hard-codes a trust model.
+//! - The verdict type is the accept/reject `bool`, pinned to the
+//!   russh hook shape at this pin (`Handler::check_server_key ->
+//!   Result<bool, Self::Error>`): the bool *is* the trust decision,
+//!   and the handler's `Err` channel stays wire/IO-failure territory.
+//!   A decline is a disposition, not an error (the crate's
+//!   error-model contract) — decline reasons are the policy's own
+//!   record, not a typed crate surface.
+//! - The [`PresentedHostKey`]'s fingerprint string is the contract-
+//!   invariant format of the crate's single fingerprint path
+//!   ([`crate::fingerprint`]) — the wave-4 dial wiring computes it
+//!   there and surfaces it here.
+//!
+//! This module is the seam and its default only: the russh
+//! client-handler consumption (`check_server_key` wiring) is wave-4
+//! dial-surface material; no dialing happens on this path yet.
+
+use async_trait::async_trait;
+
+/// The presented host key — the decision input to
+/// [`HostKeyPolicy::decide`]: the endpoint the dial contacted and the
+/// key that endpoint presented.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct PresentedHostKey {
+    /// The hostname (or IP) the dial targeted — what the assembly's
+    /// pin store or prompt labels the key against.
+    pub host: String,
+    /// The TCP port the dial targeted.
+    pub port: u16,
+    /// The presented key's fingerprint — the single fingerprint
+    /// path's `SHA256:` format ([`crate::fingerprint`]); comparison
+    /// against assembly pins or recorded entries is exact string
+    /// equality by contract.
+    pub fingerprint: String,
+}
+
+/// The trust-decision contract the dial surface calls (OQ-05): the
+/// crate surfaces the presented host key (host, port, fingerprint) to
+/// the assembly-provided policy, which decides accept or reject. The
+/// crate is policy-blind — it never decides trust itself and carries
+/// no trust model (see the module docs for the two-case model).
+///
+/// The handle shape is the family's injected-handle pattern
+/// (`Arc<dyn HostKeyPolicy>`, the alkcall `IdentityProvider`
+/// precedent): object-safe via [`async_trait`], shared by reference —
+/// learn-and-record policies hold their state by interior mutability
+/// so the same handle serves the dial and any inspection.
+///
+/// **Fail-closed posture**: when a policy cannot decide (its pin
+/// store is unavailable, its prompt failed, its metadata source is
+/// down), it must decline (`false`). The built-in default,
+/// [`RejectAll`], embodies the same posture.
+#[async_trait]
+pub trait HostKeyPolicy: Send + Sync + 'static {
+    /// Decides the trust verdict for one presented host key: `true`
+    /// accepts the key for this dial, `false` declines it — the dial
+    /// connection aborts on decline (russh's `check_server_key`
+    /// `false` → `Error::UnknownKey`, a hard, clean decline; a
+    /// mismatched pin is a hard decline, never a soft failure).
+    async fn decide(&self, presented: &PresentedHostKey) -> bool;
+}
+
+/// The built-in default host-key policy — **fail-closed**: reject
+/// every presented key.
+///
+/// russh's own reject-all default, kept as the crate's built-in
+/// default policy (`docs/architecture/auth.md` §Host-key policy). A
+/// dial that ships this policy can never connect — by design: trust
+/// is the assembly's decision to make, and the crate's default must
+/// never let a dial through on the crate's authority. There is no
+/// accept-all counterpart in the crate; the permissive case-2 policy
+/// (the ephemeral-fleet/rental shape) is the assembly's to write.
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
+pub struct RejectAll;
+
+#[async_trait]
+impl HostKeyPolicy for RejectAll {
+    async fn decide(&self, _: &PresentedHostKey) -> bool {
+        false
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{HostKeyPolicy, PresentedHostKey, RejectAll};
+    use std::sync::Arc;
+
+    fn presented(fingerprint: &str) -> PresentedHostKey {
+        PresentedHostKey {
+            host: "runner.example".into(),
+            port: 2222,
+            fingerprint: fingerprint.into(),
+        }
+    }
+
+    #[async_trait::async_trait]
+    impl HostKeyPolicy for YesAll {
+        async fn decide(&self, _: &PresentedHostKey) -> bool {
+            true
+        }
+    }
+
+    struct YesAll;
+
+    #[tokio::test]
+    async fn default_policy_rejects_everything() {
+        let policy = RejectAll;
+        assert!(!policy.decide(&presented("SHA256:unknown")).await);
+    }
+
+    #[tokio::test]
+    async fn custom_policy_verdict_is_honored() {
+        let policy = YesAll;
+        assert!(policy.decide(&presented("SHA256:any")).await);
+    }
+
+    #[tokio::test]
+    async fn dyn_handle_honors_both_verdicts() {
+        let reject: Arc<dyn HostKeyPolicy> = Arc::new(RejectAll);
+        let accept: Arc<dyn HostKeyPolicy> = Arc::new(YesAll);
+        assert!(!reject.decide(&presented("SHA256:x")).await);
+        assert!(accept.decide(&presented("SHA256:x")).await);
+    }
+
+    #[tokio::test]
+    async fn policy_receives_the_whole_presented_context() {
+        struct Recording {
+            seen: parking_lot::Mutex<Vec<PresentedHostKey>>,
+        }
+
+        #[async_trait::async_trait]
+        impl HostKeyPolicy for Recording {
+            async fn decide(&self, presented: &PresentedHostKey) -> bool {
+                self.seen.lock().push(presented.clone());
+                false
+            }
+        }
+
+        let policy = Arc::new(Recording {
+            seen: parking_lot::Mutex::new(Vec::new()),
+        });
+        policy.decide(&presented("SHA256:abc123")).await;
+        let seen = policy.seen.lock().clone();
+        assert_eq!(seen.len(), 1);
+        assert_eq!(
+            seen[0],
+            PresentedHostKey {
+                host: "runner.example".into(),
+                port: 2222,
+                fingerprint: "SHA256:abc123".into(),
+            }
+        );
+    }
+}

src/lib.rs

@@ -24,6 +24,8 @@
 
 mod error;
 mod fingerprint;
+mod host_key;
 
 pub use error::KeySourceError;
 pub use fingerprint::fingerprint;
+pub use host_key::{HostKeyPolicy, PresentedHostKey, RejectAll};

tasks/implementation/w1-host-key-policy.md

@@ -1,7 +1,7 @@
 ---
 id: w1-host-key-policy
 name: Wave 1 — the HostKeyPolicy seam (OQ-05, policy-blind)
-status: pending
+status: completed
 depends_on: [w1-manifest]
 scope: narrow
 risk: low
@@ -37,14 +37,14 @@ OQ-05 resolution) — the trust-decision contract the dial surface calls:
 
 ## Acceptance Criteria
 
-- [ ] `HostKeyPolicy` trait in place; the presented-key context
+- [x] `HostKeyPolicy` trait in place; the presented-key context
       (host, port, fingerprint) is the decision input
-- [ ] Built-in default policy rejects unknown keys (fail-closed);
+- [x] Built-in default policy rejects unknown keys (fail-closed);
       no accept-all option exists in the crate
-- [ ] Doc comments carry the two-case model and the policy-blind rule
-- [ ] Unit tests: default policy rejects; a custom policy's verdict
+- [x] Doc comments carry the two-case model and the policy-blind rule
+- [x] Unit tests: default policy rejects; a custom policy's verdict
       is honored
-- [ ] `cargo test`, `cargo clippy --all-targets -- -D warnings`,
+- [x] `cargo test`, `cargo clippy --all-targets -- -D warnings`,
       `cargo fmt --check` pass
 
 ## References
@@ -58,8 +58,59 @@ OQ-05 resolution) — the trust-decision contract the dial surface calls:
 
 ## Notes
 
-> To be filled by implementation agent
+> Landed `src/host_key.rs` (`HostKeyPolicy`, `PresentedHostKey`,
+> `RejectAll`) + lib.rs re-exports. Design decisions pinned against
+> the russh 0.65 hook shape (`Handler::check_server_key ->
+> Result<bool, Self::Error>`, async via `impl Future`, default
+> `Ok(false)` — verified in the russh 0.65.0 source):
+>
+> - **The verdict type is `bool`** (the flagged pin): the hook's bool
+>   is the trust decision and its `Err` channel is the handler's
+>   wire/IO-failure territory — a trust decline has no typed reason
+>   channel on this surface. A decline is a disposition, not an
+>   error (`src/error.rs` contract); decline reasons are the
+>   policy's own record (case-2 policies log their decisions), not a
+>   crate surface. Wave-4 wiring maps `false` → russh's hard,
+>   clean `Error::UnknownKey` decline (mismatch = hard decline,
+>   per auth.md).
+> - **The trait is async and dyn-compatible**: `#[async_trait]`,
+>   `decide(&self, &PresentedHostKey) -> bool`, `Send + Sync +
+>   'static` — the family injected-handle pattern (`Arc<dyn
+>   HostKeyPolicy>`, the alkcall `IdentityProvider`/`OperationEnv`
+>   precedent), which w1-config's dial-config handle and
+>   w1-test-doubles' inspectable double both require. State-holding
+>   (learn-and-record) policies record by interior mutability
+>   behind the shared `&self` handle.
+> - **New dep `async-trait = "0.1"`** (MIT OR Apache-2.0; the only
+>   way to get a dyn-compatible async trait at this toolchain —
+>   native AFIT is not object-safe, and russh 0.65 removed its own
+>   `async-trait` feature, per ADR-002's context). Precedented and
+>   deduped: alkcall already carries `async-trait 0.1`, so the tree
+>   adds no new transitive package. **Flagged for the wave-1 gate**
+>   (manifest addition outside w1-manifest, same class as the
+>   verdict-type pin).
+> - **`RejectAll` is the built-in default** (russh's reject-all
+>   default, kept); doc comments carry the two-case model, the
+>   policy-blind rule, the no-accept-all rule, the fail-closed
+>   disposition (an undecided policy declines), and the host-pin-is-
+>   public-config note (ADR-005 decision 6).
+> - The `PresentedHostKey.fingerprint` doc pins exact string
+>   equality against assembly pins; the string is the single
+>   fingerprint path's format, computed at wave-4 wiring.
 
 ## Summary
 
-> To be filled on completion
+> Seam landed: `src/host_key.rs` with the `HostKeyPolicy` trait
+> (`async fn decide(&self, &PresentedHostKey) -> bool`,
+> dyn-compatible via `#[async_trait]`), the `PresentedHostKey`
+> (host/port/fingerprint) decision input, and the fail-closed
+> `RejectAll` built-in default; no accept-all option exists in the
+> crate. Verdict type pinned to russh 0.65's `check_server_key`
+> shape (bool = the trust decision; decline reasons live with the
+> policy, not the crate). `async-trait = "0.1"` added for the
+> dyn-compatible async handle (alkcall-precedented, tree-deduped).
+> Tests: default rejects; custom verdict honored; both honored
+> through an `Arc<dyn Handle>`; the whole presented context reaches
+> the policy. Verified: `cargo test` (14), `cargo test
+> --all-features`, `cargo clippy --all-targets -- -D warnings`,
+> `cargo fmt --check` all pass. `taskgraph validate` clean.