- reviewer.ts: spawns separate pi agent with read-only tools
and reviewer system prompt (taskplane's review format/verdicts)
- Executor registers review_step as a custom tool on the worker session
- Worker calls review_step(step, type, step_name) at step boundaries
based on review level (0=none, 1=plan, 2=plan+code, 3=full)
- Reviewer returns APPROVE/REVISE/RETHINK with structured feedback
- REVISE feedback returned inline to worker for immediate action
- Review calls logged to task via hai task log
