feat(import/export): add tombstone support (bd-dve)

Update import/export to handle tombstones for deletion sync propagation:

Exporter:
- Include tombstones in JSONL output by setting IncludeTombstones: true
- Both single-repo and multi-repo exports now include tombstones

Importer:
- Tombstones from JSONL are imported as-is (they're issues with status=tombstone)
- Legacy deletions.jsonl entries are converted to tombstones via convertDeletionToTombstone()
- Non-tombstone issues in deletions manifest are still skipped (backward compat)
- purgeDeletedIssues() now creates tombstones instead of hard-deleting

This is Phase 2 of the tombstone implementation (bd-dli design), enabling
inline soft-delete tracking for cross-clone deletion synchronization.

🤖 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

@@ -26,8 +26,8 @@ func (s *SQLiteStorage) ExportToMultiRepo(ctx context.Context) (map[string]int,
 		return nil, nil
 	}
 
-	// Get all issues
-	allIssues, err := s.SearchIssues(ctx, "", types.IssueFilter{})
+	// Get all issues including tombstones for sync propagation (bd-dve)
+	allIssues, err := s.SearchIssues(ctx, "", types.IssueFilter{IncludeTombstones: true})
 	if err != nil {
 		return nil, fmt.Errorf("failed to query issues: %w", err)
 	}