refactor: centralize cron session metadata keys

tests/test_tool_contextvars.py

@@ -10,6 +10,7 @@ from nanobot.agent.tools.cron import CronTool
 from nanobot.agent.tools.message import MessageTool
 from nanobot.agent.tools.spawn import SpawnTool
 from nanobot.cron.service import CronService
+from nanobot.session.keys import UNIFIED_SESSION_KEY
 
 
 @pytest.mark.asyncio
@@ -262,7 +263,7 @@ async def test_webui_cron_tool_uses_unified_session_when_enabled(tmp_path) -> No
         "websocket",
         "chat-123",
         metadata={"webui": True},
-        session_key="unified:default",
+        session_key=UNIFIED_SESSION_KEY,
     )
 
     result = await tool.execute(action="add", message="standup", every_seconds=300)
@@ -270,7 +271,7 @@ async def test_webui_cron_tool_uses_unified_session_when_enabled(tmp_path) -> No
 
     jobs = tool._cron.list_jobs()
     assert len(jobs) == 1
-    assert jobs[0].payload.session_key == "unified:default"
+    assert jobs[0].payload.session_key == UNIFIED_SESSION_KEY
 
 
 @pytest.mark.asyncio