Add native macOS support with a launchd worker

[ravhello]

Motivation

The Python notifier is portable, but installation and durable rollout recovery currently support Windows Task Scheduler or Linux systemd only. macOS users therefore lack a supported continuous worker.

Scope

• Add a macOS installer or safely generalize the POSIX installer.
• Install a user LaunchAgent for the continuous worker.
• Preserve CODEX_HOME and CODEX_SQLITE_HOME.
• Match existing guarantees for private permissions, backups, rollback, idempotence, and unrelated-hook preservation.
• Add macOS doctor, uninstall, troubleshooting, and CI coverage.

Acceptance criteria

• Fresh install and upgrade work on a clean macOS user account.
• Reinstalling preserves configuration and unrelated Codex hooks.
• An unrelated LaunchAgent with the chosen label is never overwritten.
• The worker starts after login and recovers a simulated missed hook using a local fake ntfy server.
• Doctor, uninstall, and rollback are documented and tested.
• Existing Windows/Linux CI remains green and a macos-latest job passes.