fix(restart): add explicit restart mode

tests/config/test_gateway_config.py

@@ -0,0 +1,15 @@
+import pytest
+
+from nanobot.config.schema import Config, GatewayConfig
+
+
+def test_gateway_restart_mode_accepts_camel_alias():
+    config = Config.model_validate({"gateway": {"restartMode": "exit"}})
+
+    assert config.gateway.restart_mode == "exit"
+    assert config.model_dump(by_alias=True)["gateway"]["restartMode"] == "exit"
+
+
+def test_gateway_restart_mode_rejects_unknown_value():
+    with pytest.raises(ValueError):
+        GatewayConfig(restart_mode="service")