Implement GenerateMRID(prefix, branch) to generate merge request IDs
following the convention: <prefix>-mr-<hash>
- Hash derived from branch name + timestamp + random bytes for uniqueness
- Example output: gt-mr-abc123
- Includes deterministic variant for testing (GenerateMRIDWithTime)
Closes: gt-h5n.2
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
