Bump tap formula on stable tag + add macOS smoke test

[ChrisonSimtian]

Parent: #122

Release workflow bumps the formula in ChrisonSimtian/homebrew-fallout on every stable tag, and a macOS CI job runs the full smoke install path.

Definition of done

Formula update:

• Workflow opens a PR (or auto-merges) to homebrew-fallout with the new version, URL, and per-bottle SHA256s drawn from M2's release artifacts.
• brew audit --strict --online fallout-agent passes after the update.

Smoke test (macos-latest job, run on release-candidate tags + manual dispatch):

1. brew tap ChrisonSimtian/fallout && brew install fallout-agent.
2. Assert binary is signed + notarized (spctl --assess).
3. brew services start fallout-agent; assert launchd shows the daemon running.
4. Mocked coordinator handshake — agent reaches a local stub, returns "enrolled".
5. brew services stop && brew uninstall fallout-agent; assert plist is removed.
6. Job exits 0 only if all five assertions succeed.