asyncio signal handlers freeze ContextVars at install time; reinstalling
while allow_task_cancel was still False left cancel permanently disabled.
Use a process-level prompt depth counter and reinstall only after depth=0.
Max-rounds and destructive-tool prompts use asyncio.to_thread with
cancel paused on the main thread so multi-tool turns no longer look
like user cancel. Hooks may be sync or async.
NCBM