Unify OpenCode and Codex starter kits

- Add .shared/prompts/ with tool-neutral prompts
- Add .opencode/config/project.md, rules/, skills/ from opencode-kit
- Add .codex/config/project.md from codex-kit
- Copy .shared/prompts/ to both .opencode/prompts/ and .codex/prompts/
- Add features/INDEX.md and features/README.md
- Update README with unified structure

The two original starter kits are now redundant.

.codex/config/project.md

@@ -0,0 +1,32 @@
+# Codex Configuration
+
+## Project Settings
+
+This project is optimized for Codex CLI autonomous coding.
+
+## Key Files
+
+- CODEX.md - Root instructions (this file guides Codex)
+- .codex/prompts/ - Pre-built prompts for each development phase
+- features/INDEX.md - Feature tracking and status
+- docs/PRD.md - Product Requirements Document
+
+## Development Workflow
+
+1. Requirements → 2. Architecture → 3. Frontend → 4. Backend → 5. QA → 6. Deploy
+
+Each phase uses `codex exec` with natural language prompts.
+See CODEX.md for example commands.
+
+## Conventions
+
+- Feature IDs: PROJ-1, PROJ-2, etc.
+- Commits: feat(PROJ-X): description
+- Tests: co-located with source files
+- shadcn/ui first: never recreate installed components
+
+## Recommended Flags
+
+- `--full-auto` for feature development (auto-approves changes)
+- `--yolo` for quick refactoring (no sandbox)
+- Interactive (default) for review/debugging