Execute an implementation plan

Take a plan from docs/plan/ and turn it into shipped code: implement features, write tests, and validate quality.

Build Progress

Copy this checklist and track your progress:

Build Progress:
- [ ] Phase 0: Load plan and confirm scope
- [ ] Phase 1: Read context files
- [ ] Phase 2: Implement and test each task
- [ ] Phase 3: Run review agents (5 in parallel)
- [ ] Phase 4: Final validation, cleanup, and ship

Plan Input