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/tests/fixtures/sample.bin.minisig

@@ -0,0 +1,4 @@
+untrusted comment: signature from minisign secret key
+RUQKhJptuiwIkp378Z59BTwosDycAhmlhrdZZVwk1Vdb293OgcsXx0S3W0XezMtOXIXdgvQtW/DpDKlb1gdW4elQXLG5KFUgawI=
+trusted comment: timestamp:1781222247	file:sample.bin	hashed
+QtUiOfJqRKYJZTL6QV93xeLVnODr8HXWvZIR3Q1AG0yqmqesZPyiKpVa9kD34Mwp1fQ76nx1Z7c6CB1v5KHQAw==