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.

.opencode/prompts/architecture.md

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+# Architecture Prompt Template
+
+Use this prompt structure for technical design:
+
+```
+Read the feature spec at features/PROJ-X-feature-name.md.
+Design the technical architecture and add a Tech Design section to the spec file.
+
+Focus on:
+- Component structure (for frontend)
+- API design (endpoints, request/response shapes)
+- Database schema (tables, relationships, indexes)
+- Implementation order (what to build first, dependencies)
+- Security considerations
+- Tech stack decisions (libraries, patterns)
+```
+
+Example:
+```
+<codex|opencode> exec "Read features/PROJ-1-login.md. Design the auth architecture: JWT vs sessions, database schema for users, password hashing approach, and implementation order."
+```

.opencode/prompts/backend.md

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+# 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."
+```

.opencode/prompts/deploy.md

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+# Deploy Prompt Template
+
+Use this prompt structure for deployment:
+
+```
+Read the feature spec at features/PROJ-X-feature-name.md.
+Deploy the implemented feature to production.
+
+Steps:
+1. Run build/lint/tests locally
+2. Create git tag: v1.0.0-PROJ-X
+3. Push to origin
+4. Trigger deployment pipeline
+5. Verify production URL works
+6. Update feature spec with deployment status:
+
+## Deployment
+**Status:** ✅ Deployed
+**Deployed:** YYYY-MM-DD
+**Production URL:** https://...
+**Git Tag:** v1.0.0-PROJ-X
+```
+
+Example:
+```
+<codex|opencode> exec "Deploy PROJ-1-login: run npm build, create tag v1.0.0-PROJ-1, push. Verify at https://www.example.com. Update feature spec."
+```

.opencode/prompts/frontend.md

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+# Frontend Implementation Prompt Template
+
+Use this prompt structure for frontend development:
+
+```
+Read features/INDEX.md and the feature spec at features/PROJ-X-feature-name.md.
+Implement the UI components.
+
+Focus on:
+- Component structure (React/Vue components)
+- State management
+- Form handling and validation
+- Responsive design
+- Accessibility (a11y)
+- Integration with backend API
+
+Write tests co-located next to source files (e.g., LoginForm.test.tsx next to LoginForm.tsx).
+Follow the coding rules in .opencode/rules/frontend.md or .codex/config/project.md.
+```
+
+Example:
+```
+<codex|opencode> exec "Read features/PROJ-1-login.md. Build the login form component: email/password inputs, validation, submit handler, error display. Use shadcn/ui components. Write tests."
+```

.opencode/prompts/qa.md

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+# QA Prompt Template
+
+Use this prompt structure for quality assurance:
+
+```
+Read the feature spec at features/PROJ-X-feature-name.md.
+Test the implementation against each acceptance criterion.
+
+Verify:
+- [ ] Each acceptance criterion passes
+- [ ] Edge cases handled gracefully
+- [ ] Error messages are user-friendly
+- [ ] No security vulnerabilities
+- [ ] Performance is acceptable
+
+Document results in the feature spec:
+- [x] AC-1: criterion passed
+- [ ] AC-2: criterion failed (describe bug)
+
+Run: <test command from package.json or composer.json>
+
+Fix any failures before marking as complete.
+```
+
+Example:
+```
+<codex|opencode> exec "Test the login feature: verify email/password validation, test with wrong credentials, check rate limiting. Run npm test. Document results in features/PROJ-1-login.md."
+```

.opencode/prompts/requirements.md

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+# Requirements Prompt Template
+
+Use this prompt structure for creating feature specifications:
+
+```
+Read docs/<domain>/doku/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:
+```
+<ai-tool> exec "Read docs/www.example.com/doku/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."
+```