Implement 'gt mq integration status <epic>' command that displays:
- Integration branch name and creation date
- Commits ahead of main
- Merged MRs (closed, targeting integration branch)
- Pending MRs (open, targeting integration branch)
Also adds git helpers for BranchCreatedDate and CommitsAhead.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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