feat(cli): add Linux systemd auto-start alongside macOS launchd daemon

- Dispatch to launchd or systemd based on process.platform.
- New systemd.ts writes ~/.config/systemd/user/togetherlink-daemon.service,
  daemon-reloads, enables, and starts it as a user service.
- platform-auto-start.ts exposes install/uninstall/status/maybeAutoInstall
  across both platforms.
- Rename daemon subcommands: install-launchd/uninstall-launchd → install/uninstall.
- Update install.sh and help text for macOS/Linux.
- Add systemd unit tests; keep launchd tests updated.

Note: the Linux systemd path is implemented and type-checked but not yet
verified on a live Linux machine because this host is macOS.