feat: prompt caching for PLAN and REVIEW phases

[dougss]

Summary

Use Anthropic prompt caching when invoking claude -p in PLAN and REVIEW phases. The system prompt + Nexus instructions are identical across runs — caching cuts input token costs by ~90%.

Current State

• planPhase() and reviewPhase() call runClaudePipe() with static system prompts
• claude -p supports --cache-prompt flag
• No caching currently used

Plan

1. Add --cache-prompt flag to runClaudePipe() in src/integrations/claude-p/claude-pipe.ts
2. Pass from planPhase() and reviewPhase() in src/core/pipeline.ts
3. Add cache_prompt: true config in providers.anthropic section
4. Update src/config-schema.ts

Files

• src/integrations/claude-p/claude-pipe.ts
• src/core/pipeline.ts
• config/config.yaml
• src/config-schema.ts