Subagents can't determine if phase is complete. The orchestrator
(execute-plan and execute-phase commands) runs in main context,
can count plans vs summaries, and updates REQUIREMENTS.md when
phase completes.
- execute-plan: Step 2.5 updates requirements when summaries = plans
- execute-phase: Step 6 updates requirements (phase always complete)
- Removed duplicate from workflow (subagent can't run it anyway)