feat(FN-2659): add merger verification agent log events
- Emit merger agent log entries when deterministic verification starts, runs commands, and completes successfully
- Record tool_result/tool_error details for verification command outcomes including timing and truncated output summaries
- Add agent-log coverage for in-merge verification fix lifecycle events (start, retry, success, and failure)
- Extend merger deterministic verification tests to assert start/success and failure agent log entries
809a76a3c2