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/rules/frontend.md

@@ -1,17 +1,32 @@
 # Frontend Rules
 
-## HTML
+Frontend development rules for React, Next.js, Tailwind CSS, and shadcn/ui.
 
-- Semantisches HTML5
-- Accessibility beachten (a11y)
+## shadcn/ui Components
 
-## CSS
+- ALWAYS check `src/components/ui/` first before creating new components
+- NEVER recreate a component that already exists in shadcn/ui
+- If missing component: `npx shadcn@latest add <component-name>`
 
-- CSS Variables für Theming
-- Mobile-First Ansatz
-- Keine inline Styles (außer dynamisch)
+## Styling
 
-## JS
+- Use Tailwind CSS utility classes (never raw CSS)
+- Follow existing patterns in the codebase
+- Use `clsx` and `tailwind-merge` for conditional classes
 
-- Vanilla JS bevorzugt, kein Framework-Overhead
-- ES6+ Syntax
+## Component Standards
+
+- All components must be TypeScript (`.tsx`)
+- Use proper TypeScript types for props
+- Follow existing component patterns
+
+## Testing
+
+- Unit tests co-located next to source files
+- Pattern: `ComponentName.test.tsx` next to `ComponentName.tsx`
+- Use `@testing-library/react` for React tests
+
+## Lint
+
+- Run `npm run lint` after every change
+- Fix lint errors before committing