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.md

@@ -0,0 +1,117 @@
+# AI Coding Starter Kit - Codex Edition
+
+> A Next.js template with an AI-powered development workflow optimized for Codex CLI.
+
+## 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)
+```
+
+## Codex Workflow
+
+This project uses Codex CLI for autonomous coding. Run commands with `codex exec`.
+
+### Development Phases
+
+**1. Requirements**
+```bash
+codex exec "Read docs/PRD.md and features/INDEX.md. Then create a detailed feature specification for [your idea] in features/PROJ-X-feature-name.md following the structure in .codex/prompts/requirements.md"
+```
+
+**2. Architecture**
+```bash
+codex exec "Read the feature spec at features/PROJ-X-feature-name.md. Design the technical architecture and add a Tech Design section to the spec file. Focus on: component structure, API design, database schema, and implementation order."
+```
+
+**3. Frontend**
+```bash
+codex exec "Read features/INDEX.md and the feature spec. Implement the UI components using React, Next.js, Tailwind CSS, and shadcn/ui. Install missing shadcn components with: npx shadcn@latest add <component-name>. Write tests next to source files."
+```
+
+**4. Backend**
+```bash
+codex exec "Read the feature spec Tech Design section. Build the API endpoints, database migrations, and RLS policies. Use Supabase client from src/lib/supabase.ts. Write tests for all endpoints."
+```
+
+**5. QA**
+```bash
+codex exec "Read the feature spec's acceptance criteria. Test the implementation against each criterion. Run: npm test for unit tests, npm run test:e2e for E2E tests. Fix any failures."
+```
+
+**6. Deploy**
+```bash
+codex exec "Deploy to Vercel: vercel --prod. Run production checks: npm run build, npm run lint. Verify deployment at the provided URL."
+```
+
+## Feature Tracking
+
+All features tracked in `features/INDEX.md`. 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
+- **Tests:** Unit tests co-located next to source files (`useHook.test.ts` next to `useHook.ts`)
+
+## 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
+```
+
+## Codex Modes
+
+| Mode | Use Case |
+|------|----------|
+| `--full-auto` | Autonomous work with auto-approval (recommended for feature development) |
+| `--yolo` | No sandbox, no approvals (fastest, use for refactoring/quick fixes) |
+| Default (interactive) | Ask for approval before each change |
+
+## Example Workflows
+
+### Create new feature from scratch
+```bash
+codex exec --full-auto "Initialize the project: read docs/PRD.md to understand the project, then create a feature spec for the login feature in features/PROJ-1-login.md"
+```
+
+### Implement and test a feature
+```bash
+codex exec --full-auto "Implement the feature described in features/PROJ-1-login.md. Build the UI, backend, and tests. Verify with npm test."
+```
+
+### Review and fix issues
+```bash
+codex exec --full-auto "Run npm test and npm run lint. Fix all issues found."
+```
+
+## Prompts Directory
+
+See `.codex/prompts/` for pre-built prompts for each development phase.