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/requirements.md

@@ -0,0 +1,25 @@
+# Requirements Prompt Template
+
+Use this prompt structure with Codex:
+
+```
+Read docs/PRD.md to understand the project vision and constraints.
+Read features/INDEX.md to see existing features and the next available PROJ-X ID.
+
+[Describe the feature idea clearly]
+
+Create a feature specification in features/PROJ-X-feature-name.md with:
+- Feature name and ID
+- User stories (3-5, with: As a [role], I want [action], so that [benefit])
+- Acceptance criteria (testable, specific)
+- Edge cases (error handling, edge inputs, boundary conditions)
+- Dependencies (other features required)
+- Implementation notes (optional)
+
+Update features/INDEX.md with the new feature status.
+```
+
+Example:
+```
+codex exec "Read docs/PRD.md and features/INDEX.md. Create a feature spec for user authentication with email/password login, including: user stories for login/logout, acceptance criteria for valid/invalid credentials, edge cases for locked accounts and rate limiting."
+```