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

CLAUDE.md.old

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+# AI Coding Starter Kit
+
+> A Next.js template with an AI-powered development workflow using specialized skills for Requirements, Architecture, Frontend, Backend, QA, and Deployment.
+
+## Tech Stack
+
+- **Framework:** Next.js 16 (App Router), TypeScript
+- **Styling:** Tailwind CSS + shadcn/ui (copy-paste components)
+- **Backend:** Supabase (PostgreSQL + Auth + Storage) - optional
+- **Deployment:** Vercel
+- **Validation:** Zod + react-hook-form
+- **State:** React useState / Context API
+
+## Project Structure
+
+```
+src/
+  app/              Pages (Next.js App Router)
+  components/
+    ui/             shadcn/ui components (NEVER recreate these)
+  hooks/            Custom React hooks
+  lib/              Utilities (supabase.ts, utils.ts)
+features/           Feature specifications (PROJ-X-name.md)
+  INDEX.md          Feature status overview
+docs/
+  PRD.md            Product Requirements Document
+  production/       Production guides (Sentry, security, performance)
+```
+
+## Development Workflow
+
+1. `/requirements` - Create feature spec from idea
+2. `/architecture` - Design tech architecture (PM-friendly, no code)
+3. `/frontend` - Build UI components (shadcn/ui first!)
+4. `/backend` - Build APIs, database, RLS policies
+5. `/qa` - Test against acceptance criteria + security audit
+6. `/deploy` - Deploy to Vercel + production-ready checks
+
+## Feature Tracking
+
+All features tracked in `features/INDEX.md`. Every skill reads it at start and updates it when done. Feature specs live in `features/PROJ-X-name.md`.
+
+## Key Conventions
+
+- **Feature IDs:** PROJ-1, PROJ-2, etc. (sequential)
+- **Commits:** `feat(PROJ-X): description`, `fix(PROJ-X): description`
+- **Single Responsibility:** One feature per spec file
+- **shadcn/ui first:** NEVER create custom versions of installed shadcn components
+- **Human-in-the-loop:** All workflows have user approval checkpoints
+- **Tests:** Unit tests co-located next to source files (`useHook.test.ts` next to `useHook.ts`). E2E tests in `tests/`.
+
+## Build & Test Commands
+
+```bash
+npm run dev          # Development server (localhost:3000)
+npm run build        # Production build
+npm run lint         # ESLint
+npm run start        # Production server
+npm test             # Vitest unit/integration tests
+npm run test:e2e     # Playwright E2E tests
+npm run test:all     # Both test suites
+```
+
+## Product Context
+
+@docs/PRD.md
+
+## Feature Overview
+
+@features/INDEX.md