cmd_update only ran 'systemctl --user restart hermes-gateway', which
left manually-started gateway processes alive, causing duplicates.
Now uses get_running_pid() from gateway/status.py (scoped to
HERMES_HOME) to find and SIGTERM this installation's gateway before
restarting. Safe with multiple Hermes installations since each
HERMES_HOME has its own PID file.
If no systemd service exists, informs the user to restart manually.
Based on PR #1131 by teknium1. Dropped the cli.py Rich from_ansi
changes (already on main).