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

.gitignore

@@ -37,3 +37,7 @@ yarn-error.log*
 # typescript
 *.tsbuildinfo
 next-env.d.ts
+
+# Codex
+.codex/data/
+.codex/cache/