feat(error): land the fail-closed key-source error taxonomy

- KeySourceError with exactly five variants (Locked, Absent,
  Decryption, UnknownKeyVersion, Decode) — ADR-005 decision 4's
  failure matrix as distinct shapes, the vault taxonomy reshaped
  never collapsed; no catch-all, no Other/Unknown
- payload shapes pinned: Locked/Absent payload-free;
  Decryption/UnknownKeyVersion carry the vault's opaque messages;
  Decode carries a message so the RC-pinned ssh-key error type
  never enters the public API (flagged for the wave-1 gate)
- each variant doc-commented with ADR-005 semantics + fail-closed
  consequence; module stays engine-blind (no vault/russh types)
- no From<VaultServiceError>: non-mappable vault variants would
  force a collapse; w1-key-sources maps explicitly at its seam
- exhaustive-match + Display/Debug/Clone unit tests

Verified: cargo test (default + all-features), cargo clippy
--all-targets --all-features -- -D warnings, cargo fmt --check,
cargo doc --no-deps, taskgraph validate

tasks/implementation/w1-error-model.md

@@ -1,7 +1,7 @@
 ---
 id: w1-error-model
 name: Wave 1 — the error model (fail-closed taxonomy)
-status: pending
+status: completed
 depends_on: [w1-manifest]
 scope: narrow
 risk: low
@@ -39,13 +39,13 @@ message is the honest content), flagged for the wave-1 gate.
 
 ## Acceptance Criteria
 
-- [ ] `KeySourceError` has exactly the five pinned variants; no
+- [x] `KeySourceError` has exactly the five pinned variants; no
       catch-all, no `Other`/`Unknown`
-- [ ] Doc comments state each variant's ADR-005 semantics and its
+- [x] Doc comments state each variant's ADR-005 semantics and its
       fail-closed consequence
-- [ ] Unit tests construct every variant and assert `Display`/`Debug`
+- [x] Unit tests construct every variant and assert `Display`/`Debug`
       paths compile clean
-- [ ] `cargo test`, `cargo clippy --all-targets -- -D warnings`,
+- [x] `cargo test`, `cargo clippy --all-targets -- -D warnings`,
       `cargo fmt --check` pass
 
 ## References
@@ -56,8 +56,43 @@ message is the honest content), flagged for the wave-1 gate.
 
 ## Notes
 
-> To be filled by implementation agent
+> Payload shapes pinned (flagged for the wave-1 gate): `Locked` and
+> `Absent` are payload-free — no honest content beyond the variant
+> itself (the caller knows which source it requested);
+> `Decryption(String)` and `UnknownKeyVersion(String)` carry the
+> vault's opaque messages (`VaultServiceError::Encryption` /
+> `InvalidPath` arrive as strings per POC-2 §3 — string is the honest
+> content). `Decode` carries `String` rather than the typed
+> `ssh_key::Error` (`russh::keys::decode_secret_key`'s error type):
+> the ssh-key RC type is deliberately kept out of the published
+> public API — when the RC settles the payload can be re-typed, but
+> freezing an RC error type now would be a worse one-way door. The
+> module doc comment records the deliberately-not-error classes
+> (auth declines = dispositions per ADR-004; wire decline reasons =
+> wave-2; config validation = none unless w1-config finds a need —
+> the OQ-08 fields decline at request time). No `From<VaultServiceError>`
+> blanket impl on purpose: alkvault's taxonomy has variants
+> (`Mnemonic`, `Derivation`, `AlreadyUnlocked`, `UnsupportedKeyType`)
+> that do not map onto the five without collapsing, and the ADR's
+> reshape-never-collapse rule kills the conversion; w1-key-sources
+> maps explicitly at its seam. The module stays engine-blind — no
+> alkvault or russh types referenced at the error layer. Tests: an
+> exhaustive `match` (no wildcard — the closed-variant invariant is
+> compile-time enforced) plus `Display` string assertions over all
+> five variants, `Debug` output, and `Clone` payload carrying.
 
 ## Summary
 
-> To be filled on completion
+> `src/error.rs` landed: `KeySourceError` with exactly five variants
+> (`Locked`, `Absent`, `Decryption(String)`, `UnknownKeyVersion(String)`,
+> `Decode(String)`), each doc-commented with its ADR-005 semantics and
+> fail-closed consequence (locked → no dial + no server; absent/decryption/
+> unknown-version/decode → no dial; host-key path → no server
+> construction throughout). Re-exported from the crate root
+> (`mod error; pub use error::KeySourceError;` — the alktty pattern).
+> No catch-all, no `Other/Unknown`, no blanket `From` from the vault's
+> taxonomy. Four unit tests (exhaustive match over the enum,
+> `Display`, `Debug`, `Clone`). Verified: `cargo test` (4/4 + doc
+> tests), `cargo test --all-features`, `cargo clippy --all-targets
+> -- -D warnings` (default + all-features), `cargo fmt --check`,
+> `cargo doc --no-deps` (no warnings), `taskgraph validate` clean.