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/types/types.go

@@ -222,8 +222,9 @@ type BlockedIssue struct {
 // TreeNode represents a node in a dependency tree
 type TreeNode struct {
 	Issue
-	Depth     int  `json:"depth"`
-	Truncated bool `json:"truncated"`
+	Depth     int    `json:"depth"`
+	ParentID  string `json:"parent_id"`
+	Truncated bool   `json:"truncated"`
 }
 
 // Statistics provides aggregate metrics