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>

cmd/bd/dep_test.go

@@ -1,9 +1,13 @@
 package main
 
 import (
+	"bytes"
 	"context"
 	"fmt"
+	"io"
+	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -266,3 +270,217 @@ func TestDepRemove(t *testing.T) {
 		t.Errorf("Expected 0 dependencies after removal, got %d", len(deps))
 	}
 }
+
+func TestDepTreeFormatFlag(t *testing.T) {
+	// Test that the --format flag exists on depTreeCmd
+	flag := depTreeCmd.Flags().Lookup("format")
+	if flag == nil {
+		t.Fatal("depTreeCmd should have --format flag")
+	}
+
+	// Test default value is empty string
+	if flag.DefValue != "" {
+		t.Errorf("Expected default format='', got %q", flag.DefValue)
+	}
+
+	// Test usage text mentions mermaid
+	if !strings.Contains(flag.Usage, "mermaid") {
+		t.Errorf("Expected flag usage to mention 'mermaid', got %q", flag.Usage)
+	}
+}
+
+func TestGetStatusEmoji(t *testing.T) {
+	tests := []struct {
+		status types.Status
+		want   string
+	}{
+		{types.StatusOpen, "☐"},
+		{types.StatusInProgress, "◧"},
+		{types.StatusBlocked, "⚠"},
+		{types.StatusClosed, "☑"},
+		{types.Status("unknown"), "?"},
+	}
+
+	for _, tt := range tests {
+		t.Run(string(tt.status), func(t *testing.T) {
+			got := getStatusEmoji(tt.status)
+			if got != tt.want {
+				t.Errorf("getStatusEmoji(%q) = %q, want %q", tt.status, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestOutputMermaidTree(t *testing.T) {
+	tests := []struct {
+		name   string
+		tree   []*types.TreeNode
+		rootID string
+		want   []string // Lines that must appear in output
+	}{
+		{
+			name:   "empty tree",
+			tree:   []*types.TreeNode{},
+			rootID: "test-1",
+			want: []string{
+				"flowchart TD",
+				`test-1["No dependencies"]`,
+			},
+		},
+		{
+			name: "single dependency",
+			tree: []*types.TreeNode{
+				{
+					Issue:    types.Issue{ID: "test-1", Title: "Task 1", Status: types.StatusInProgress},
+					Depth:    0,
+					ParentID: "",
+				},
+				{
+					Issue:    types.Issue{ID: "test-2", Title: "Task 2", Status: types.StatusClosed},
+					Depth:    1,
+					ParentID: "test-1",
+				},
+			},
+			rootID: "test-1",
+			want: []string{
+				"flowchart TD",
+				`test-1["◧ test-1: Task 1"]`,
+				`test-2["☑ test-2: Task 2"]`,
+				"test-1 --> test-2",
+			},
+		},
+		{
+			name: "multiple dependencies",
+			tree: []*types.TreeNode{
+				{
+					Issue:    types.Issue{ID: "test-1", Title: "Main", Status: types.StatusOpen},
+					Depth:    0,
+					ParentID: "",
+				},
+				{
+					Issue:    types.Issue{ID: "test-2", Title: "Sub 1", Status: types.StatusClosed},
+					Depth:    1,
+					ParentID: "test-1",
+				},
+				{
+					Issue:    types.Issue{ID: "test-3", Title: "Sub 2", Status: types.StatusBlocked},
+					Depth:    1,
+					ParentID: "test-1",
+				},
+			},
+			rootID: "test-1",
+			want: []string{
+				"flowchart TD",
+				`test-1["☐ test-1: Main"]`,
+				`test-2["☑ test-2: Sub 1"]`,
+				`test-3["⚠ test-3: Sub 2"]`,
+				"test-1 --> test-2",
+				"test-1 --> test-3",
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Capture stdout
+			old := os.Stdout
+			r, w, _ := os.Pipe()
+			os.Stdout = w
+
+			outputMermaidTree(tt.tree, tt.rootID)
+
+			w.Close()
+			os.Stdout = old
+
+			var buf bytes.Buffer
+			io.Copy(&buf, r)
+			output := buf.String()
+
+			// Verify all expected lines appear
+			for _, line := range tt.want {
+				if !strings.Contains(output, line) {
+					t.Errorf("expected output to contain %q, got:\n%s", line, output)
+				}
+			}
+		})
+	}
+}
+
+func TestOutputMermaidTree_Siblings(t *testing.T) {
+	// Test case: Siblings with children (reproduces issue with wrong parent inference)
+	// Structure:
+	//   BD-1 (root)
+	//   ├── BD-2 (sibling 1)
+	//   │   └── BD-4 (child of BD-2)
+	//   └── BD-3 (sibling 2)
+	//       └── BD-5 (child of BD-3)
+	tree := []*types.TreeNode{
+		{
+			Issue:    types.Issue{ID: "BD-1", Title: "Parent", Status: types.StatusOpen},
+			Depth:    0,
+			ParentID: "",
+		},
+		{
+			Issue:    types.Issue{ID: "BD-2", Title: "Sibling 1", Status: types.StatusOpen},
+			Depth:    1,
+			ParentID: "BD-1",
+		},
+		{
+			Issue:    types.Issue{ID: "BD-3", Title: "Sibling 2", Status: types.StatusOpen},
+			Depth:    1,
+			ParentID: "BD-1",
+		},
+		{
+			Issue:    types.Issue{ID: "BD-4", Title: "Child of Sibling 1", Status: types.StatusOpen},
+			Depth:    2,
+			ParentID: "BD-2",
+		},
+		{
+			Issue:    types.Issue{ID: "BD-5", Title: "Child of Sibling 2", Status: types.StatusOpen},
+			Depth:    2,
+			ParentID: "BD-3",
+		},
+	}
+
+	// Capture stdout
+	old := os.Stdout
+	r, w, _ := os.Pipe()
+	os.Stdout = w
+
+	outputMermaidTree(tree, "BD-1")
+
+	w.Close()
+	os.Stdout = old
+
+	var buf bytes.Buffer
+	io.Copy(&buf, r)
+	output := buf.String()
+
+	// Verify correct edges exist
+	correctEdges := []string{
+		"BD-1 --> BD-2",
+		"BD-1 --> BD-3",
+		"BD-2 --> BD-4",
+		"BD-3 --> BD-5",
+	}
+
+	for _, edge := range correctEdges {
+		if !strings.Contains(output, edge) {
+			t.Errorf("expected edge %q to be present, got:\n%s", edge, output)
+		}
+	}
+
+	// Verify incorrect edges do NOT exist (siblings shouldn't be connected)
+	incorrectEdges := []string{
+		"BD-2 --> BD-3",   // Siblings shouldn't be connected
+		"BD-3 --> BD-4",   // BD-4's parent is BD-2, not BD-3
+		"BD-4 --> BD-3",   // Wrong direction
+		"BD-4 --> BD-5",   // These are cousins, not parent-child
+	}
+
+	for _, edge := range incorrectEdges {
+		if strings.Contains(output, edge) {
+			t.Errorf("incorrect edge %q should NOT be present, got:\n%s", edge, output)
+		}
+	}
+}