feat(host-agent): Phase 3a signed self-update (minisign) + CI signing gate

Agent only ever runs a binary whose minisign signature verifies against
the EMBEDDED public key. NATS host.cmd func 'update' {url}: download
binary + .minisig from the CDN -> verify against embedded pubkey ->
atomic swap (.old rollback) -> relaunch. URL allowlist (https + cdn.
corrosionmgmt.com only, rejects userinfo-bypass), 100MiB cap. Closes the
supply-chain hole: even a malicious CDN upload can't run unsigned.

CI: build-host-agent.yml signs every artifact with MINISIGN_SECRET_KEY
(Gitea secret) and publishes .minisig alongside; the step FAILS the
build if the secret is absent (refuses to ship unsigned). Bumped to
alpha.6.

6 deterministic tests (accept valid / reject tampered+garbage+empty sig,
URL allowlist incl userinfo-bypass, atomic swap+rollback). Fixtures
signed with the real release key so tests need no key at runtime. Full
suite 50/50 green; musl + native build clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

corrosion-host-agent/PROTOCOL.md

@@ -85,6 +85,7 @@ Request: `{ "func": "<name>" }`. Reply: `{ "status": "success" | "error", ... }`
 | `ping`    | `version`, `commit`, `uptime_seconds`                     |
 | `probe`   | `report` — fresh ProbeReport (also cached for heartbeat)  |
 | `sysinfo` | `snapshot` — full heartbeat payload, collected on demand  |
+| `update`  | `{ "func": "update", "url": "https://cdn.corrosionmgmt.com/host-agent/.../corrosion-host-agent-<plat>" }` → downloads the binary + `<url>.minisig`, verifies the minisign signature against the agent's EMBEDDED public key, atomically swaps (with `.old` rollback), replies `{ status: success, message: "...relaunching" }`, then relaunches the new binary. Rejects anything not signed by the release key and any URL that isn't `https://cdn.corrosionmgmt.com`. |
 
 Unknown funcs return `status: "error"` with a message listing supported funcs.