fix(inbox): drop unused batch_fetcher = None after end-of-batch drain

Lint nit from review bot — _drain_uploads() runs and the function
immediately advances to the cursor save + return, so the local
re-assign is dead code.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

workspace/inbox.py

@@ -641,7 +641,6 @@ def _poll_once(
     # would race with the still-running fetches.
     if batch_fetcher is not None:
         _drain_uploads(batch_fetcher)
-        batch_fetcher = None
 
     if last_id is not None:
         state.save_cursor(last_id, cursor_key)