feat(claude-code): persistent signing key makes the manifest verifiable

[imran-siddique]

Found while doing a full end-to-end run of the plugin as a new user would (/manifest + /trace lifecycle in a throwaway home).

The gap

The signed Agent Manifest could not be verified by anyone. sign_all minted a fresh keypair per run with generate_ed25519() and discarded it; manifest.json stored only key_id (a hash of the public key), never the key. The signature was real but unverifiable after the process exited, which contradicts the README's "shareable proof a third party can verify." (TRACE was fine: trace.json embeds cnf.jwk and verifies standalone.)

The fix

• Persistent key. One Ed25519 key at ~/.claude/agentrust/signing_key.json, generated once and reused every run (reconstructed from the stored seed via cryptography), best-effort owner-only perms. Records now carry a stable signing identity instead of a throwaway key.
• Published public key. Each run writes verification_key.json ({key_id, public_key_b64url}) beside the records. The private half never leaves the machine.
• Docs. Corrected the README claim and added the exact manifest-verification recipe (load verification_key.json into trusted_keys); /trace command doc updated; .gitignored the new artifacts.

Validation

• Key is stable across runs (same key_id).
• A third party with only verification_key.json -> OverallResult.VALID, signature_verified True.
• Tamper agent_id / an artifact hash, or use a wrong key -> MISMATCH, signature_verified False.
• TRACE suite still PASS (8/8); full /manifest + /trace E2E still green.
• 2 crypto-guarded regression tests added (importorskip); 15 pass both with and without the crypto packages. ruff clean.

Note

Trace records still use a per-run key from agentrust_trace (self-verifiable via cnf.jwk). Reusing the one persistent identity for TRACE too is a reasonable follow-up but out of scope here. Also flagged separately: the plugin's pytest suite isn't run by CI (validate.yml only checks integration.yaml schemas) — worth wiring up.

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