Implement 'gt mq integration create <epic>' command to create integration
branches for batch work on epics. The command:
1. Verifies the epic exists in beads
2. Creates branch integration/<epic-id> from origin/main
3. Pushes the branch to origin
4. Stores integration branch info in the epic's metadata
Also adds helper methods to git package:
- CreateBranchFrom: create branch from specific ref
- BranchExists: check if local branch exists
- RemoteBranchExists: check if branch exists on remote
Future MRs for the epic's children can target the integration branch
with: gt mq submit --epic <epic-id>
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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