refactor: Migrate from Claude Code to Codex CLI - .claude to .codex restructure

- Replaced .claude/ with .codex/ directory structure
- Created CODEX.md as root instructions for Codex
- Added .codex/prompts/ with templates for each development phase
- Added .codex/config/project.md for project configuration
- Removed .claude/agents/ (not compatible with Codex)
- Updated features/INDEX.md and docs/PRD.md references

.codex/prompts/qa.md

@@ -0,0 +1,20 @@
+# QA Prompt Template
+
+Use this prompt structure with Codex:
+
+```
+Read the feature spec at features/PROJ-X-feature-name.md, especially:
+- Acceptance criteria
+- User stories
+- Edge cases
+
+Test the implementation:
+1. Run unit tests: npm test
+2. Run E2E tests: npm run test:e2e
+3. Manual testing if needed
+4. Check each acceptance criterion
+5. Verify edge cases are handled
+
+Report: what passed, what failed, what needs fixing.
+Fix all failures before declaring done.
+```