fix(cleanup): resolve CHECK constraint failure and add tombstone pruning

- Fix bd-tnsq: executeDelete now sets closed_at=NULL when creating
  tombstones, satisfying the CHECK constraint that requires
  closed_at IS NULL when status != 'closed'

- Fix bd-08ea: cleanup command now also prunes expired tombstones
  (older than 30 days) after converting closed issues to tombstones

- Add regression test for batch deletion of closed issues

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/sqlite/queries.go

@@ -1202,12 +1202,15 @@ func (s *SQLiteStorage) executeDelete(ctx context.Context, tx *sql.Tx, inClause
 	_ = rows.Close()
 
 	// 3. Convert issues to tombstones (only for issues that exist)
+	// Note: closed_at must be set to NULL because of CHECK constraint:
+	// (status = 'closed') = (closed_at IS NOT NULL)
 	now := time.Now()
 	deletedCount := 0
 	for id, originalType := range issueTypes {
 		execResult, err := tx.ExecContext(ctx, `
 			UPDATE issues
 			SET status = ?,
+			    closed_at = NULL,
 			    deleted_at = ?,
 			    deleted_by = ?,
 			    delete_reason = ?,