Implement success handling for the merge queue Engineer:
- Add handleSuccess method that handles successful merge completion
- Update MR body with merge_commit SHA and close_reason
- Close MR with 'merged' reason
- Close source issue with reference to MR ID
- Delete source branch if delete_merged_branches is configured
- Add DeleteRemoteBranch method to git package
- Add git client to Engineer struct
- Add tests for new functionality
Closes gt-3x1.5
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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