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/memory/memory.go

@@ -616,6 +616,7 @@ func (m *MemoryStorage) SetJSONLFileHash(ctx context.Context, fileHash string) e
 // GetDependencyTree gets the dependency tree for an issue
 func (m *MemoryStorage) GetDependencyTree(ctx context.Context, issueID string, maxDepth int, showAllPaths bool, reverse bool) ([]*types.TreeNode, error) {
 	// Simplified implementation - just return direct dependencies
+	// Note: reverse parameter is accepted for interface compatibility but not fully implemented in memory storage
 	deps, err := m.GetDependencies(ctx, issueID)
 	if err != nil {
 		return nil, err