Implement E14-T2 start-work execution backend

[dmoliveira]

Summary

• add scripts/start_work_command.py to parse/validate markdown plan artifacts, execute ordered checklist steps with checkpoint transitions, persist latest run status, and report deviations
• wire /start-work command aliases in opencode.json, add installer self-check/hints, and extend install smoke coverage for start-work execution + status/deviations
• extend selftest coverage for valid/invalid plan execution flows and update roadmap/docs/changelog for Epic 14 Task 14.2 completion

Testing

• make validate
• make selftest
• make install-test