The detached bash subprocess spawned by /restart gets killed by
systemd's KillMode=mixed cgroup cleanup, leaving the gateway dead.
Under systemd (detected via INVOCATION_ID env var), /restart now uses
via_service=True which exits with code 75 — RestartForceExitStatus=75
in the unit file makes systemd auto-restart the service. The detached
subprocess approach is preserved as fallback for non-systemd
environments (Docker, tmux, foreground mode).
When a user sends /restart, the gateway now persists their routing info
(platform, chat_id, thread_id) to .restart_notify.json. After the new
gateway process starts and adapters connect, it reads the file, sends a
'Gateway restarted successfully' message to that specific chat, and
cleans up the file.
This follows the same pattern as _send_update_notification (used by
/update). Thread IDs are preserved so the notification lands in the
correct Telegram topic or Discord thread.
Previously, after /restart the user had no feedback that the gateway was
back — they had to send a message to find out. Now they get a proactive
notification and know their session continues.
