Formula installs launchd plist via brew services

Parent: #122

The Homebrew formula (__M1__) declares a `service` block so `brew services start fallout-agent` registers a launchd daemon under `/Library/LaunchDaemons/com.fallout.agent.plist` (system-level) — not a per-user agent.

## Definition of done

- Formula's `service do` block defines: `run`, `keep_alive`, `log_path`, `error_log_path`, `working_dir`, and `require_root true` (system daemon, not user agent).
- `brew services start fallout-agent` registers and starts the daemon.
- Daemon survives reboot in a test VM.
- `brew services stop fallout-agent` stops cleanly; `brew services restart` works.
- Plist label is `com.fallout.agent` (no underscores; consistent with bundle ID conventions).