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>

cmd/bd/export.go

@@ -346,6 +346,16 @@ Examples:
 			}
 		}
 
+		// 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(issues))
+		for _, issue := range issues {
+			if !issue.Ephemeral {
+				filtered = append(filtered, issue)
+			}
+		}
+		issues = filtered
+
 		// Sort by ID for consistent output
 		sort.Slice(issues, func(i, j int) bool {
 			return issues[i].ID < issues[j].ID