Implements priority scoring for merge queue ordering:
## Changes to gt mq list
- Add SCORE column showing priority score (higher = process first)
- Sort MRs by score descending instead of simple priority
- Add CONVOY column showing convoy ID if tracked
## New gt mq next command
- Returns highest-score MR ready for processing
- Supports --strategy=fifo for FIFO ordering fallback
- Supports --quiet for just printing MR ID
- Supports --json for programmatic access
## Changes to Refinery
- Queue() now sorts by priority score instead of simple priority
- Uses ScoreMR from mrqueue package for consistent scoring
## MR Fields Extended
- Added retry_count, last_conflict_sha, conflict_task_id
- Added convoy_id, convoy_created_at for convoy tracking
- These fields feed into priority scoring function
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>