fix(subagent): isolate subagent from parent run checkpointer (#3559)

Subagent _create_agent() now passes checkpointer=False to prevent
inheriting the parent run's synchronous checkpointer via copy_context(),
which would cause NotImplementedError when aget_tuple() is called on
the async path. Subagents are one-shot delegations that never resume,
so persistence is unnecessary.

backend/packages/harness/deerflow/subagents/executor.py

@@ -351,6 +351,7 @@ class SubagentExecutor:
             middleware=middlewares,
             system_prompt=None,
             state_schema=ThreadState,
+            checkpointer=False,
         )
 
     async def _load_skills(self) -> list[Skill]: