refactor: centralize cron session metadata keys

tests/agent/test_runner_injections.py

@@ -592,8 +592,8 @@ async def test_waiting_dispatch_does_not_replace_active_pending_queue(tmp_path):
 @pytest.mark.asyncio
 async def test_followup_routed_to_pending_queue(tmp_path):
     """Unified-session follow-ups should route into the active pending queue."""
-    from nanobot.agent.loop import UNIFIED_SESSION_KEY
     from nanobot.bus.events import InboundMessage
+    from nanobot.session.keys import UNIFIED_SESSION_KEY
 
     loop = _make_loop(tmp_path)
     loop._unified_session = True