The _watch_update_progress() poll loop never deleted .update_prompt.json
after forwarding the prompt to the user, causing the same prompt to be
re-sent every poll cycle (2s). Two fixes:
1. Delete .update_prompt.json after forwarding — the update process only
polls for .update_response, it doesn't need the prompt file to persist.
2. Guard re-sends with _update_prompt_pending check — belt-and-suspenders
to prevent duplicates even under race conditions.
Add regression test asserting the prompt is sent exactly once.
4eecaf06e4