fix(storage): normalize timestamps to UTC to prevent validation failures (#1123)

All time.Now() calls in the dolt storage layer now use time.Now().UTC()
to ensure consistent timezone handling. Previously, timestamps could be
stored with mixed timezone formats (UTC 'Z' vs local '+01:00'), causing
bv validation to fail when updated_at appeared earlier than created_at
in absolute time.

Files modified:
- transaction.go: CreateIssue, UpdateIssue, CloseIssue
- issues.go: CreateIssue, CreateIssues, UpdateIssue, CloseIssue, markDirty, manageClosedAt
- rename.go: UpdateIssueID (2 locations)
- events.go: AddIssueComment (2 locations)
- dirty.go: SetExportHash
- queries.go: Overdue filter, GetStaleIssues

Fixes: bd-84gw9

Co-authored-by: LoomDeBWiles <loomenwiles@gmail.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/dolt/events.go

@@ -68,7 +68,7 @@ func (s *DoltStore) AddIssueComment(ctx context.Context, issueID, author, text s
 	result, err := s.db.ExecContext(ctx, `
 		INSERT INTO comments (issue_id, author, text, created_at)
 		VALUES (?, ?, ?, ?)
-	`, issueID, author, text, time.Now())
+	`, issueID, author, text, time.Now().UTC())
 	if err != nil {
 		return nil, fmt.Errorf("failed to add comment: %w", err)
 	}
@@ -83,7 +83,7 @@ func (s *DoltStore) AddIssueComment(ctx context.Context, issueID, author, text s
 		IssueID:   issueID,
 		Author:    author,
 		Text:      text,
-		CreatedAt: time.Now(),
+		CreatedAt: time.Now().UTC(),
 	}, nil
 }