fix(gateway): /restart uses service restart under systemd instead of detached subprocess

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).

tests/gateway/test_restart_notification.py

@@ -47,6 +47,48 @@ async def test_restart_command_writes_notify_file(tmp_path, monkeypatch):
     assert "thread_id" not in data  # no thread → omitted
 
 
+@pytest.mark.asyncio
+async def test_restart_command_uses_service_restart_under_systemd(tmp_path, monkeypatch):
+    """Under systemd (INVOCATION_ID set), /restart uses via_service=True."""
+    monkeypatch.setattr(gateway_run, "_hermes_home", tmp_path)
+    monkeypatch.setenv("INVOCATION_ID", "abc123")
+
+    runner, _adapter = make_restart_runner()
+    runner.request_restart = MagicMock(return_value=True)
+
+    source = make_restart_source(chat_id="42")
+    event = MessageEvent(
+        text="/restart",
+        message_type=MessageType.TEXT,
+        source=source,
+        message_id="m1",
+    )
+
+    await runner._handle_restart_command(event)
+    runner.request_restart.assert_called_once_with(detached=False, via_service=True)
+
+
+@pytest.mark.asyncio
+async def test_restart_command_uses_detached_without_systemd(tmp_path, monkeypatch):
+    """Without systemd, /restart uses the detached subprocess approach."""
+    monkeypatch.setattr(gateway_run, "_hermes_home", tmp_path)
+    monkeypatch.delenv("INVOCATION_ID", raising=False)
+
+    runner, _adapter = make_restart_runner()
+    runner.request_restart = MagicMock(return_value=True)
+
+    source = make_restart_source(chat_id="42")
+    event = MessageEvent(
+        text="/restart",
+        message_type=MessageType.TEXT,
+        source=source,
+        message_id="m1",
+    )
+
+    await runner._handle_restart_command(event)
+    runner.request_restart.assert_called_once_with(detached=True, via_service=False)
+
+
 @pytest.mark.asyncio
 async def test_restart_command_preserves_thread_id(tmp_path, monkeypatch):
     """Thread ID is saved when the requester is in a threaded chat."""