feat: add Mermaid.js format for dependency tree visualization (#191)

Adds `bd dep tree --format mermaid` to export dependency trees as Mermaid.js flowcharts.

Features:
- Status indicators: ☐ open, ◧ in_progress, ⚠ blocked, ☑ closed
- Theme-agnostic design
- Works with --reverse flag
- Comprehensive unit tests following TDD

Co-authored-by: David Laing <david@davidlaing.com>

internal/storage/sqlite/dependencies.go

@@ -587,7 +587,7 @@ func (s *SQLiteStorage) GetDependencyTree(ctx context.Context, issueID string, m
 		if err != nil {
 			return nil, fmt.Errorf("failed to scan tree node: %w", err)
 		}
-		_ = parentID // Silence unused variable warning
+		node.ParentID = parentID
 
 		if closedAt.Valid {
 			node.ClosedAt = &closedAt.Time