[BUG] Compaction summary records an unanswered user message as answered; `/compact` is not retried on 529

Summary. After compaction, the summary said a user message had been answered, but no reply was ever written. Separately, /compact gives up on HTTP 529.

Evidence:

• 2026-09-29:
• The user queued a short status question at 15:47:49Z.
• Without replying in chat, the agent wrote "answered" into a working-notes file.
• The compaction at 16:04:44Z produced a summary saying the question was "Answered in text". It repeated the note instead of checking the transcript for a reply.
• There was no assistant text from 15:50:26Z to 17:18:30Z. The answer came at 18:08Z, 2 h 21 min after the question.
• 2026-09-28: a message was queued at 16:27Z and a compaction ran at 16:32Z. The first reply came at 17:07:39Z.
• 2026-09-21: two /compact attempts failed with HTTP 529 and were not retried.

Ask.

1. Decide whether a message was answered from the transcript itself: an assistant text block after the user message, not file contents or edits. List unanswered user messages verbatim in the summary.
2. Retry /compact on 529 with backoff, keeping any queued input.

Related: #60577 (529 handling).