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
