fix(gateway): use kickstart -k for atomic launchd restart

Replace the two-step stop/start restart with a single
launchctl kickstart -k call. When the gateway triggers a
restart from inside its own process tree, the old stop
command kills the shell before the start half is reached.
kickstart -k lets launchd handle the kill+restart atomically.

tests/hermes_cli/test_gateway_service.py

@@ -297,7 +297,7 @@ class TestGatewaySystemServiceRouting:
             gateway_cli,
             "launchd_restart",
             lambda: (_ for _ in ()).throw(
-                gateway_cli.subprocess.CalledProcessError(5, ["launchctl", "start", "ai.hermes.gateway"])
+                gateway_cli.subprocess.CalledProcessError(5, ["launchctl", "kickstart", "-k", "gui/501/ai.hermes.gateway"])
             ),
         )