docs: Add Phase 002 overview and reorganize design docs

[Nicered]

Summary

• Reorganize design documents into phase-based structure (phase-001/, phase-002/)
• Add 5 new feature design documents for Phase 001:
  • 010-architect-feature.md - Code review feature
  • 011-checkpoint-feature.md - Git-based checkpoints
  • 012-testing-feature.md - E2E testing with Playwright
  • 013-project-context-feature.md - PROJECT.md management
  • 014-env-feature.md - Environment variable management
• Create Phase 002 overview document with API test results
• Update CLAUDE.md with phase-based documentation rules

Test plan

• Verify all documentation files exist and are properly linked
• Confirm pnpm build succeeds
• Review Phase 002 overview accurately reflects current feature status

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