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/frontend.md

@@ -0,0 +1,21 @@
+# Frontend Prompt Template
+
+Use this prompt structure with Codex:
+
+```
+Read features/INDEX.md and the feature spec at features/PROJ-X-feature-name.md.
+Check existing components: ls src/components/ui/
+Check installed shadcn: ls src/components/ui/ | wc -l
+
+[Describe what to build]
+
+Implementation rules:
+- ALWAYS use shadcn/ui components (check src/components/ui/ first)
+- If missing: npx shadcn@latest add <component-name>
+- Tailwind CSS for all styling
+- Write unit tests next to source files (*.test.ts next to *.ts)
+- Run npm run lint after changes
+
+If design files exist: check design/ mockups/ assets/
+Otherwise ask about: visual style, brand colors, layout preference.
+```