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