feat: Add markdown file support to bd create command

Implement `bd create -f file.md` to parse markdown files and create
multiple issues in one command. This enables drafting features in
markdown and converting them to tracked issues.

Features:
- Parse markdown H2 headers (##) as issue titles
- Support all issue fields via H3 sections (### Priority, ### Type, etc.)
- Handle multiple issues per file
- Comprehensive validation and error handling
- Full test coverage with 5 test cases

Closes bd-91 (GH-9)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/markdown_test.go

@@ -0,0 +1,238 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestParseMarkdownFile(t *testing.T) {
+	tests := []struct {
+		name     string
+		content  string
+		expected []*IssueTemplate
+		wantErr  bool
+	}{
+		{
+			name: "simple issue",
+			content: `## Fix authentication bug
+
+This is a critical bug in the auth system.
+
+### Priority
+1
+
+### Type
+bug
+`,
+			expected: []*IssueTemplate{
+				{
+					Title:       "Fix authentication bug",
+					Description: "This is a critical bug in the auth system.",
+					Priority:    1,
+					IssueType:   "bug",
+				},
+			},
+		},
+		{
+			name: "multiple issues",
+			content: `## First Issue
+
+Description for first issue.
+
+### Priority
+0
+
+### Type
+feature
+
+## Second Issue
+
+Description for second issue.
+
+### Priority
+2
+
+### Type
+task
+`,
+			expected: []*IssueTemplate{
+				{
+					Title:       "First Issue",
+					Description: "Description for first issue.",
+					Priority:    0,
+					IssueType:   "feature",
+				},
+				{
+					Title:       "Second Issue",
+					Description: "Description for second issue.",
+					Priority:    2,
+					IssueType:   "task",
+				},
+			},
+		},
+		{
+			name: "issue with all fields",
+			content: `## Comprehensive Issue
+
+Initial description text.
+
+### Priority
+1
+
+### Type
+feature
+
+### Description
+Detailed description here.
+Multi-line support.
+
+### Design
+Design notes go here.
+
+### Acceptance Criteria
+- Must do this
+- Must do that
+
+### Assignee
+alice
+
+### Labels
+backend, urgent
+
+### Dependencies
+bd-10, bd-20
+`,
+			expected: []*IssueTemplate{
+				{
+					Title:              "Comprehensive Issue",
+					Description:        "Detailed description here.\nMulti-line support.",
+					Design:             "Design notes go here.",
+					AcceptanceCriteria: "- Must do this\n- Must do that",
+					Priority:           1,
+					IssueType:          "feature",
+					Assignee:           "alice",
+					Labels:             []string{"backend", "urgent"},
+					Dependencies:       []string{"bd-10", "bd-20"},
+				},
+			},
+		},
+		{
+			name: "dependencies with types",
+			content: `## Issue with typed dependencies
+
+### Priority
+2
+
+### Type
+task
+
+### Dependencies
+blocks:bd-10, discovered-from:bd-20
+`,
+			expected: []*IssueTemplate{
+				{
+					Title:        "Issue with typed dependencies",
+					Priority:     2,
+					IssueType:    "task",
+					Dependencies: []string{"blocks:bd-10", "discovered-from:bd-20"},
+				},
+			},
+		},
+		{
+			name: "default values",
+			content: `## Minimal Issue
+
+Just a title and description.
+`,
+			expected: []*IssueTemplate{
+				{
+					Title:       "Minimal Issue",
+					Description: "Just a title and description.",
+					Priority:    2,    // default
+					IssueType:   "task", // default
+				},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Create temp file
+			tmpDir := t.TempDir()
+			tmpFile := filepath.Join(tmpDir, "test.md")
+			if err := os.WriteFile(tmpFile, []byte(tt.content), 0600); err != nil {
+				t.Fatalf("Failed to create test file: %v", err)
+			}
+
+			// Parse file
+			got, err := parseMarkdownFile(tmpFile)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("parseMarkdownFile() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+
+			if len(got) != len(tt.expected) {
+				t.Errorf("parseMarkdownFile() got %d issues, want %d", len(got), len(tt.expected))
+				return
+			}
+
+			// Compare each issue
+			for i, gotIssue := range got {
+				wantIssue := tt.expected[i]
+
+				if gotIssue.Title != wantIssue.Title {
+					t.Errorf("Issue %d: Title = %q, want %q", i, gotIssue.Title, wantIssue.Title)
+				}
+				if gotIssue.Description != wantIssue.Description {
+					t.Errorf("Issue %d: Description = %q, want %q", i, gotIssue.Description, wantIssue.Description)
+				}
+				if gotIssue.Priority != wantIssue.Priority {
+					t.Errorf("Issue %d: Priority = %d, want %d", i, gotIssue.Priority, wantIssue.Priority)
+				}
+				if gotIssue.IssueType != wantIssue.IssueType {
+					t.Errorf("Issue %d: IssueType = %q, want %q", i, gotIssue.IssueType, wantIssue.IssueType)
+				}
+				if gotIssue.Design != wantIssue.Design {
+					t.Errorf("Issue %d: Design = %q, want %q", i, gotIssue.Design, wantIssue.Design)
+				}
+				if gotIssue.AcceptanceCriteria != wantIssue.AcceptanceCriteria {
+					t.Errorf("Issue %d: AcceptanceCriteria = %q, want %q", i, gotIssue.AcceptanceCriteria, wantIssue.AcceptanceCriteria)
+				}
+				if gotIssue.Assignee != wantIssue.Assignee {
+					t.Errorf("Issue %d: Assignee = %q, want %q", i, gotIssue.Assignee, wantIssue.Assignee)
+				}
+
+				// Compare slices
+				if !stringSlicesEqual(gotIssue.Labels, wantIssue.Labels) {
+					t.Errorf("Issue %d: Labels = %v, want %v", i, gotIssue.Labels, wantIssue.Labels)
+				}
+				if !stringSlicesEqual(gotIssue.Dependencies, wantIssue.Dependencies) {
+					t.Errorf("Issue %d: Dependencies = %v, want %v", i, gotIssue.Dependencies, wantIssue.Dependencies)
+				}
+			}
+		})
+	}
+}
+
+func TestParseMarkdownFile_FileNotFound(t *testing.T) {
+	_, err := parseMarkdownFile("/nonexistent/file.md")
+	if err == nil {
+		t.Error("Expected error for non-existent file, got nil")
+	}
+}
+
+func stringSlicesEqual(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	if len(a) == 0 && len(b) == 0 {
+		return true
+	}
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+	return true
+}