fix(pr-bot): announce a bot-made merge once, cap automatic retries, show why a review failed

Observed on the first live merge (#383 via the Telegram button): runConfirmed
announced the merge and the scan five seconds later announced it again as a
closed PR. The scan now stays quiet for PRs the bot itself merged or closed,
and a merge of a `merge-with-fixes` verdict reminds that merging applies none
of the listed fixes.

A failed review used to be re-queued on every scan with no limit (two PRs
failed once each and were retried fine, but a head that keeps failing would
cost a session every ten minutes forever): three failures on one head now
stop the automatic retries until /review N or a new push. The failure notice
carries the reviewer's last message, so "finished without writing
report.json" says what it wrote instead.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

scripts/pr-bot/state.ts

@@ -41,6 +41,9 @@ export interface PrRecord {
   worktreeDir?: string;
   telegramMessageId?: number;
   lastError?: string;
+  /** Consecutive failed attempts at `failedSha`; the scan stops auto-retrying at MAX_AUTO_RETRIES. */
+  failedAttempts?: number;
+  failedSha?: string;
   closedAs?: 'merged' | 'closed';
   updatedAt: string;
 }