fix: Store D-Bus agent instance globally to prevent GC

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

main.py

@@ -17,6 +17,7 @@ from dbus_next import Variant
 
 
 bus = None
+agent_instance = None
 
 # --- Bluetooth constants and agent ---
 BLUEZ_SERVICE = 'org.bluez'
@@ -104,8 +105,9 @@ async def get_adapter():
     return None
 
 async def register_agent():
-    agent = Agent(AGENT_IFACE)
-    bus.export(AGENT_PATH, agent)
+    global agent_instance
+    agent_instance = Agent(AGENT_IFACE)
+    bus.export(AGENT_PATH, agent_instance)
 
     introspection = await bus.introspect(BLUEZ_SERVICE, '/org/bluez')
     manager_obj = bus.get_proxy_object(BLUEZ_SERVICE, '/org/bluez', introspection)