fix(tasks): keep scheduled-task prompt cache stable

Move scheduled-task current-time context out of the system prompt and into a user-role context message so the system prompt remains stable for prompt caching. Preserve time grounding on both the agent-loop path and fallback direct-call path, with focused regression coverage.

tests/test_task_shell_tools.py

@@ -111,7 +111,8 @@ async def test_scheduled_task_honors_global_disabled_tools(monkeypatch):
     captured = {}
 
     async def _capture(endpoint_url, model, task, session_id, *,
-                       system_prompt=None, disabled_tools=None, relevant_tools=None):
+                       system_prompt=None, disabled_tools=None, relevant_tools=None,
+                       datetime_context_msg=None):
         captured["disabled_tools"] = disabled_tools
         captured["relevant_tools"] = relevant_tools
         return "done"