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/architecture.md

@@ -0,0 +1,16 @@
+# Architecture Prompt Template
+
+Use this prompt structure with Codex:
+
+```
+Read the feature spec at features/PROJ-X-feature-name.md.
+
+Design the technical architecture and add a "Tech Design" section to the spec with:
+- Component hierarchy (list components to build)
+- API endpoints (method, path, request/response shapes)
+- Database schema changes (tables, columns, RLS policies)
+- Implementation order (what to build first, dependencies)
+- Tech stack decisions ( libraries, patterns)
+
+Focus on: practical, implementable, PM-friendly (no code yet).
+```