feat(mol): filter ephemeral issues from JSONL export (bd-687g)

Ephemeral issues should never be exported to issues.jsonl. They exist only
in SQLite and are shared via .beads/redirect pointers. This prevents
"zombie" issues from resurrecting after mol squash deletes them.

Changes:
- Filter ephemeral issues in autoflush, export, and multirepo_export
- Add --summary flag to bd mol squash for agent-provided summaries
- Fix DeleteIssue to also remove comments (missing cascade)
- Add tests for ephemeral filtering and comment deletion

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

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

internal/storage/sqlite/queries.go

@@ -1093,6 +1093,12 @@ func (s *SQLiteStorage) DeleteIssue(ctx context.Context, id string) error {
 		return fmt.Errorf("failed to delete events: %w", err)
 	}
 
+	// Delete comments (no FK cascade on this table) (bd-687g)
+	_, err = tx.ExecContext(ctx, `DELETE FROM comments WHERE issue_id = ?`, id)
+	if err != nil {
+		return fmt.Errorf("failed to delete comments: %w", err)
+	}
+
 	// Delete from dirty_issues
 	_, err = tx.ExecContext(ctx, `DELETE FROM dirty_issues WHERE issue_id = ?`, id)
 	if err != nil {