* fix(threads): remove a deleted thread's files off the event loop
DELETE /api/threads/{id} called shutil.rmtree over the thread directory
directly on the event loop, so deleting a thread with a large workspace
stalled every other Gateway request and live SSE stream until the whole
tree was gone. Branch copying in the same router already offloads its
filesystem work through run_file_io; deletion did not.
The removal now runs on the file-IO pool. It is wrapped in await_drained
rather than bare-awaited: cancelling the await abandons the worker, not
the removal, so the durable delete reservation would otherwise release
while files are still being deleted, and a newly admitted run could
write into a directory that is about to disappear.
* docs(changelog): reference #6319 in the thread delete off-loop entry
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
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