feat(FN-840): add closed/merged PR feedback contract and clean up unused code
- Add closed/merged PR feedback contract to pr-monitor for post-merge follow-up behavior
- Fix PR monitor drainComments + consume/drain behavior with comprehensive tests
- Add PR feedback follow-up tests and scheduler follow-up logic
- Remove unused activity log, agent log viewer, and multi-agent log code from dashboard
- Remove unused store methods, types, API endpoints, and route handlers
- Update README docs to document manual-merge PR follow-up behavior and fix auth wording
- Add dashboard CLI test coverage for new commands
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