feat(beads): add MR field parsing for merge-request issues
Implement ParseMRFields and SetMRFields helper functions to extract
and update structured fields in merge-request issue descriptions.
Fields supported: branch, target, source_issue, worker, rig,
merge_commit, close_reason.
Features:
- Case-insensitive key matching
- Alternate key formats (snake_case, kebab-case)
- Preserves non-MR prose content
- Handles mixed MR fields and prose
- Round-trip safe (parse then format preserves data)
Closes: gt-h5n.1
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
f011e9bc80