feat: enforce agent routing and persist restart state

Task 4: stale room blocking + agent_id binding
- MatrixBot._check_agent_routing: blocks normal messages when user has no
  selected agent or room is bound to a different agent
- agent_routing_enabled flag on MatrixRuntime activates the check only
  in real multi-agent mode (RoutedPlatformClient)
- make_handle_new_chat now writes agent_id into new room metadata when
  user already has a selected agent

Task 5: durable restart state tests
- test_restart_persistence.py proves selected_agent_id, room agent_id,
  platform_chat_id, and the sequence counter all survive SQLiteStore
  close/reopen; also covers clean startup with no prior state

adapter/matrix/handlers/chat.py

@@ -8,6 +8,7 @@ from nio.api import RoomVisibility
 from nio.responses import RoomCreateError
 
 from adapter.matrix.store import (
+    get_selected_agent_id,
     get_user_meta,
     next_chat_id,
     next_platform_chat_id,
@@ -104,18 +105,18 @@ def make_handle_new_chat(
             state_key=room_id,
         )
 
-        await set_room_meta(
-            store,
-            room_id,
-            {
-                "room_type": "chat",
-                "chat_id": chat_id,
-                "display_name": room_name,
-                "matrix_user_id": event.user_id,
-                "space_id": space_id,
-                "platform_chat_id": platform_chat_id,
-            },
-        )
+        selected_agent_id = await get_selected_agent_id(store, event.user_id)
+        room_meta: dict = {
+            "room_type": "chat",
+            "chat_id": chat_id,
+            "display_name": room_name,
+            "matrix_user_id": event.user_id,
+            "space_id": space_id,
+            "platform_chat_id": platform_chat_id,
+        }
+        if selected_agent_id:
+            room_meta["agent_id"] = selected_agent_id
+        await set_room_meta(store, room_id, room_meta)
         ctx = await chat_mgr.get_or_create(
             user_id=event.user_id,
             chat_id=chat_id,