Implement the actual merge execution in Engineer.ProcessMR:
- ExecuteMerge function: checkout target, merge with --no-ff, run tests
- pushWithRetry: exponential backoff retry logic (3 retries, 1s base delay)
- runTests: execute configurable test command
- gitRun/gitOutput: helper methods for git operations
When tests fail, resets to HEAD~1 before returning failure.
When push fails after retries, resets to HEAD~1 before returning failure.
Conflict detection during merge triggers automatic merge --abort.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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