fix(workflow): prevent circular duplicate closure in auto-close script

- Skip closing an issue as duplicate of another that is already closed
  (avoids circular closure when bot and human close in opposite order).
- Skip when duplicate target is self (same issue number).
- Extract testable helpers: isDupeComment, isDupeCommentOldEnough,
  authorDisagreedWithDupe, getLastDupeComment, decideAutoClose.
- Add 23 unit tests (Bun) and run them in CI before auto-close step.
- Add scripts/AUTO_CLOSE_DUPLICATES_CROSS_VERIFY.md for impact summary.

Co-authored-by: Cursor <cursoragent@cursor.com>

.github/workflows/auto-close-duplicates.yml

@@ -22,6 +22,9 @@ jobs:
         with:
           bun-version: latest
 
+      - name: Run auto-close-duplicates tests
+        run: bun test scripts/auto-close-duplicates
+
       - name: Auto-close duplicate issues
         run: bun run scripts/auto-close-duplicates.ts
         env: