Fix bd-ar2 code review issues: metadata tracking and multi-repo support

This commit addresses critical code review findings from bd-dvd and bd-ymj fixes:

## Completed Tasks

### bd-ar2.1: Extract duplicated metadata update code
- Created `updateExportMetadata()` helper function
- Eliminated 22-line duplication between createExportFunc and createSyncFunc
- Single source of truth for metadata updates

### bd-ar2.2: Add multi-repo support to export metadata updates
- Added per-repo metadata key tracking with keySuffix parameter
- Both export and sync functions now update metadata for all repos

### bd-ar2.3: Fix tests to use actual daemon functions
- TestExportUpdatesMetadata now calls updateExportMetadata() directly
- Added TestUpdateExportMetadataMultiRepo() for multi-repo testing
- Fixed export_mtime_test.go tests to call updateExportMetadata()

### bd-ar2.9: Fix variable shadowing in GetNextChildID
- Changed `err` to `resurrectErr` to avoid shadowing
- Improves code clarity and passes linter checks

### bd-ar2.10: Fix hasJSONLChanged to support per-repo keys
- Updated hasJSONLChanged() to accept keySuffix parameter
- Reads metadata with correct per-repo keys
- All callers updated (validatePreExport, daemon import, sync command)

### bd-ar2.11: Use stable repo identifiers instead of paths
- Added getRepoKeyForPath() helper function
- Uses stable identifiers like ".", "../frontend" instead of absolute paths
- Metadata keys now portable across machines and clones
- Prevents orphaned metadata when repos are moved

## Files Changed
- cmd/bd/daemon_sync.go: Helper functions, metadata updates
- cmd/bd/integrity.go: hasJSONLChanged() with keySuffix support
- cmd/bd/sync.go: Updated to use getRepoKeyForPath()
- cmd/bd/*_test.go: Tests updated for new signatures
- internal/storage/sqlite/hash_ids.go: Fixed variable shadowing

## Testing
All export, sync, and integrity tests pass.

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

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

internal/storage/sqlite/hash_ids.go

@@ -35,9 +35,9 @@ func (s *SQLiteStorage) GetNextChildID(ctx context.Context, parentID string) (st
 	}
 	if count == 0 {
 		// Try to resurrect parent from JSONL history before failing (bd-dvd fix)
-		resurrected, err := s.TryResurrectParent(ctx, parentID)
-		if err != nil {
-			return "", fmt.Errorf("failed to resurrect parent %s: %w", parentID, err)
+		resurrected, resurrectErr := s.TryResurrectParent(ctx, parentID)
+		if resurrectErr != nil {
+			return "", fmt.Errorf("failed to resurrect parent %s: %w", parentID, resurrectErr)
 		}
 		if !resurrected {
 			return "", fmt.Errorf("parent issue %s does not exist and could not be resurrected from JSONL history", parentID)