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/multirepo_export.go

@@ -50,6 +50,16 @@ func (s *SQLiteStorage) ExportToMultiRepo(ctx context.Context) (map[string]int,
 		issue.Labels = labels
 	}
 
+	// Filter out ephemeral issues - they should never be exported to JSONL (bd-687g)
+	// Ephemeral issues exist only in SQLite and are shared via .beads/redirect, not JSONL.
+	filtered := make([]*types.Issue, 0, len(allIssues))
+	for _, issue := range allIssues {
+		if !issue.Ephemeral {
+			filtered = append(filtered, issue)
+		}
+	}
+	allIssues = filtered
+
 	// Group issues by source_repo
 	issuesByRepo := make(map[string][]*types.Issue)
 	for _, issue := range allIssues {