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/prompts/backend.md

@@ -0,0 +1,23 @@
+# Backend Implementation Prompt Template
+
+Use this prompt structure for backend development:
+
+```
+Read features/INDEX.md and the feature spec at features/PROJ-X-feature-name.md.
+Implement the backend components.
+
+Focus on:
+- API endpoints (create, read, update, delete)
+- Database queries (SQL, migrations)
+- Business logic (validation, error handling)
+- Authentication/Authorization if needed
+- Rate limiting and security headers
+
+Write tests in the appropriate tests/ directory.
+Follow the coding rules in .opencode/rules/ or .codex/config/project.md.
+```
+
+Example:
+```
+<codex|opencode> exec "Read features/PROJ-1-login.md. Implement the login API: POST /auth/login endpoint, password verification, JWT generation. Add migration for users table. Write PHPUnit tests."
+```