feat(cli): auto-start togetherlink daemon via launchd on macOS

Adds a macOS launchd user agent (com.togetherlink.daemon) so the shared
proxy daemon starts at login and is kept alive, fixing "Connection
refused" errors after reboot when ChatGPT Desktop tries to reach the
local proxy.

- New  for plist
  generation/install/uninstall/status.
- New commands: .
- One-time silent auto-install for existing installed bundles on macOS.
-  installs the agent on Darwin after bundle verification.
- Added deterministic tests for plist generation and auto-install decision.

Closes: session request