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>

CLAUDE.md

@@ -20,6 +20,9 @@ bd create "Issue title" -t bug|feature|task -p 0-4 -d "Description" --json
 # Create with explicit ID (for parallel workers)
 bd create "Issue title" --id worker1-100 -p 1 --json
 
+# Create multiple issues from markdown file
+bd create -f feature-plan.md --json
+
 # Update issue status
 bd update <id> --status in_progress --json