Add MergeQueueConfig struct and RigConfig type with:
- All merge queue settings (enabled, target_branch, on_conflict, etc.)
- Default values via DefaultMergeQueueConfig()
- Validation for on_conflict strategy and poll_interval duration
- Load/Save/Validate functions following existing config patterns
- Comprehensive tests for round-trip, custom config, and validation
Implements gt-h5n.8.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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