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/backend.md

@@ -0,0 +1,19 @@
+# Backend Prompt Template
+
+Use this prompt structure with Codex:
+
+```
+Read the feature spec Tech Design section at features/PROJ-X-feature-name.md.
+Check existing APIs: ls src/app/api/
+Check Supabase schema: cat src/lib/supabase.ts
+
+[Describe what to build]
+
+Implementation rules:
+- Use Supabase client from src/lib/supabase.ts
+- Create API routes in src/app/api/[entity]/
+- Follow existing patterns for route structure
+- Add RLS policies for new tables
+- Write tests for all endpoints
+- Run: npm test after implementation
+```