Add subcommands so the desktop agent runs in the background on login — the real
desktop product shape, no window. `tether-agent install` writes a launchd
LaunchAgent (macOS) or systemd user unit (Linux) with the resolved config baked
in (run mode + server/account/name/room), loads it, and it survives reboots;
`uninstall`/`status` manage it. Foreground `run` unchanged (the no-subcommand
default). Windows falls through with a hint.
Verified on macOS: install loads the agent (launchctl lists it, process runs
with the baked args), uninstall removes it cleanly.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
