macOS: auto-reinstall Cmd+; hotkey listener on update

Already-configured users had a LaunchAgent that pins the old binary path and
runs the old (broken, never-firing) listener until reloaded, and auto-install
was skipped for them. Add a versioned migration so updates fix everyone:

- Add hotkey_listener_version to setup state (HOTKEY_LISTENER_VERSION = 1).
- On launch: fresh users auto-install (unchanged); configured users on an older
  listener version get the LaunchAgent reinstalled (re-points to current binary,
  restarts the listener) without re-running setup; preserves their terminal.
- Extract mac_hotkey_action_for_state as a pure, unit-tested decision function.