Decompose architecture into 35 atomic tasks across 10 generations for implementation

tasks/auth/client-auth-handler.md

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+---
+id: auth/client-auth-handler
+name: Implement client-side SSH authentication with Ed25519 key pairs
+status: pending
+depends_on:
+  - auth/key-loading
+  - auth/error-types
+scope: narrow
+risk: low
+impact: component
+level: implementation
+---
+
+## Description
+
+Implement the client-side SSH authentication. The client presents an Ed25519 private key during SSH handshake. This creates the `russh::client::Handler` implementation and the `russh::client::ConnectStreamConfig` that uses the loaded key.
+
+No password auth. The client handler is simpler than the server — it just needs to provide the private key and handle the auth callback from russh.
+
+## Acceptance Criteria
+
+- [ ] `crates/wraith-core/src/auth/client_auth.rs` exports `ClientAuthConfig` and client handler
+- [ ] `ClientAuthConfig` holds: `private_key: KeyPair`, optional `public_key: PublicKey`
+- [ ] `ClientAuthConfig::from_key_source(source: KeySource) -> Result<Self>` — loads key via key-loading module
+- [ ] Implements `russh::client::Handler` with `auth_publickey()` returning the public key
+- [ ] Client handler returns `russh::client::AuthResult::Accept` or appropriate auth state
+- [ ] Unit tests: valid key creates handler, auth flow succeeds with mock SSH session
+
+## References
+
+- docs/architecture/client.md — "Authentication is Ed25519 public key or OpenSSH certificate (ADR-012)"
+- docs/architecture/decisions/012-auth-ed25519-and-cert-authority.md — key-based auth only
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion

tasks/auth/error-types.md

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+---
+id: auth/error-types
+name: Define error types for transport, auth, channel, and configuration layers
+status: pending
+depends_on:
+  - setup/project-init
+scope: narrow
+risk: trivial
+impact: phase
+level: implementation
+---
+
+## Description
+
+Define the error hierarchy per the overview.md layered error pattern:
+- **Transport errors** — connection failures, TLS handshake failures, iroh endpoint errors
+- **Auth errors** — key rejection, certificate validation failures, missing keys
+- **Channel errors** — target unreachable, proxy failure
+- **Config errors** — invalid flags, key file not found, bind failure
+
+Use `thiserror` for structured error types propagated via `anyhow::Result` in the public API. The key design: transport/auth errors cause reconnection (client) or rejection (server). Channel-level errors close that channel without killing the session.
+
+## Acceptance Criteria
+
+- [ ] `crates/wraith-core/src/error.rs` exports error types
+- [ ] `TransportError` enum: `ConnectionFailed`, `HandshakeFailed`, `Timeout`, `ProxyFailed`
+- [ ] `AuthError` enum: `KeyRejected`, `CertInvalid`, `CertExpired`, `CertPrincipalMismatch`, `NoMatchingKey`
+- [ ] `ChannelError` enum: `TargetUnreachable`, `ProxyConnectFailed`, `ChannelClosed`
+- [ ] `ConfigError` enum: `InvalidFlag`, `KeyFileNotFound`, `BindFailed`, `IncompatibleOptions`
+- [ ] All error types implement `std::error::Error` via `thiserror`, `Display`, and `Debug`
+- [ ] Error types have `source` chaining where appropriate (e.g., `TransportError::HandshakeFailed { source: std::io::Error }`)
+- [ ] Re-exported from `crates/wraith-core/src/lib.rs`
+- [ ] Unit tests for Display output of each error variant
+
+## References
+
+- docs/architecture/overview.md — "Error handling follows a consistent layered pattern"
+- docs/architecture/client.md — error handling section (transport → reconnect, channel → close)
+- docs/architecture/server.md — error handling section
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion

tasks/auth/key-loading.md

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+---
+id: auth/key-loading
+name: Implement SSH key material loading (file paths and in-memory data)
+status: pending
+depends_on:
+  - auth/error-types
+  - setup/project-init
+scope: narrow
+risk: low
+impact: component
+level: implementation
+---
+
+## Description
+
+Implement key material loading that accepts both file paths and in-memory data per the programmatic-first API (ADR-011). Key inputs (`--identity`, `--authorized-keys`, `--cert-authority`, `--key`) accept either:
+- **File path**: load from filesystem
+- **In-memory data**: raw key bytes provided programmatically
+
+All keys must be in **OpenSSH key format** (not PEM/PKCS#1/PKCS#8). This module handles:
+- Loading private keys (OpenSSH format: `-----BEGIN OPENSSH PRIVATE KEY-----`)
+- Loading public keys (OpenSSH format: `ssh-ed25519 AAAA... user@host`)
+- Loading authorized_keys files (standard OpenSSH format)
+- Parsing `cert-authority` entries in authorized_keys
+
+## Acceptance Criteria
+
+- [ ] `crates/wraith-core/src/auth/keys.rs` exports key loading functions
+- [ ] `KeySource` enum: `File(PathBuf)` and `Memory(Vec<u8>)` for unified key input handling
+- [ ] `load_private_key(source: KeySource) -> Result<russh::key::KeyPair>` — loads OpenSSH private key from file or memory
+- [ ] `load_public_keys(source: KeySource) -> Result<Vec<russh::key::PublicKey>>` — loads one or more public keys from authorized_keys format
+- [ ] Parses standard `authorized_keys` format including options (e.g., `cert-authority,permit-port-forwarding ssh-ed25519 AAAA...`)
+- [ ] `CertAuthorityEntry` struct: `public_key: PublicKey, options: Vec<String>` parsed from authorized_keys cert-authority lines
+- [ ] Returns `ConfigError::KeyFileNotFound` for missing file paths
+- [ ] Returns `ConfigError::InvalidFlag` with clear message for PEM-encoded (non-OpenSSH) keys
+- [ ] Unit tests: load Ed25519 key from file, load from memory, parse authorized_keys with multiple entries, reject PEM format
+
+## References
+
+- docs/architecture/client.md — Key Material Format section
+- docs/architecture/server.md — Key Material Format section
+- docs/architecture/decisions/012-auth-ed25519-and-cert-authority.md — authorized_keys format with cert-authority
+- docs/architecture/decisions/011-no-ssh-config-programmatic-api.md — programmatic-first, file paths or in-memory
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion

tasks/auth/server-auth-handler.md

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+---
+id: auth/server-auth-handler
+name: Implement server-side authentication (Ed25519 keys + OpenSSH cert-authority)
+status: pending
+depends_on:
+  - auth/key-loading
+  - auth/error-types
+scope: moderate
+risk: medium
+impact: component
+level: implementation
+---
+
+## Description
+
+Implement the server-side SSH authentication logic per ADR-012:
+
+1. **Ed25519 public key**: `auth_publickey()` checks presented key against the authorized set using constant-time comparison
+2. **OpenSSH certificate authority**: validates presented certificate — checks CA signature, expiry, and principal restrictions (`permit-port-forwarding`, `no-pty`, `source-address`)
+
+No password authentication over SSH. This is the `russh::server::Handler::auth_publickey()` implementation that the server handler will call.
+
+## Acceptance Criteria
+
+- [ ] `crates/wraith-core/src/auth/server_auth.rs` exports `ServerAuthConfig` and auth logic
+- [ ] `ServerAuthConfig` holds: `authorized_keys: HashSet<PublicKey>`, `cert_authorities: Vec<CertAuthorityEntry>`
+- [ ] `ServerAuthConfig::from_keys_and_ca()` constructor: loads authorized keys and cert-authority entries from provided key sources
+- [ ] Auth check function: given a presented key/certificate, return `Accept` or `Reject`
+- [ ] Ed25519 key matching uses constant-time comparison (via `russh`/`ssh-key` crate builtins)
+- [ ] Certificate validation checks: CA signature valid, cert not expired, principal restrictions enforced
+- [ ] Certificate options respected: `permit-port-forwarding`, `no-pty`, `source-address`
+- [ ] Returns `AuthError::KeyRejected` or `AuthError::CertInvalid`/`CertExpired`/`CertPrincipalMismatch` on failure
+- [ ] Unit tests: valid key accepted, invalid key rejected, cert-authority signed cert accepted, expired cert rejected, wrong principal rejected
+
+## References
+
+- docs/architecture/server.md — Authentication section
+- docs/architecture/decisions/012-auth-ed25519-and-cert-authority.md — ADR for key + cert-authority
+- docs/architecture/client.md — "Authentication is Ed25519 public key or OpenSSH certificate"
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion