feat: introduce Radix UI design system skill with a component template and examples.

skills/radix-ui-design-system/examples/README.md

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+# Radix UI Design System - Skill Examples
+
+This folder contains practical examples demonstrating how to use Radix UI primitives to build accessible, customizable components.
+
+## Examples
+
+### `dialog-example.tsx`
+
+Demonstrates Dialog (Modal) component patterns:
+- **BasicDialog**: Standard modal with form
+- **ControlledDialog**: Externally controlled modal state
+
+**Key Concepts**:
+- Portal rendering outside DOM hierarchy
+- Overlay (backdrop) handling
+- Accessibility requirements (Title, Description)
+- Custom styling with CSS
+
+### `dropdown-example.tsx`
+
+Complete dropdown menu implementation:
+- **CompleteDropdown**: Full-featured menu with all Radix primitives
+  - Regular items
+  - Separators and labels
+  - Checkbox items
+  - Radio groups
+  - Sub-menus
+- **ActionsMenu**: Simple actions menu for data tables/cards
+
+**Key Concepts**:
+- Compound component architecture
+- Keyboard navigation
+- Item indicators (checkboxes, radio buttons)
+- Nested sub-menus
+
+## Usage
+
+```tsx
+import { BasicDialog } from './examples/dialog-example';
+import { CompleteDropdown } from './examples/dropdown-example';
+
+function App() {
+  return (
+    <>
+      <BasicDialog />
+      <CompleteDropdown />
+    </>
+  );
+}
+```
+
+## Styling
+
+These examples use CSS classes. You can:
+1. Copy the CSS from each file
+2. Replace with Tailwind classes
+3. Use CSS-in-JS (Stitches, Emotion, etc.)
+
+## Learn More
+
+- [Main SKILL.md](../SKILL.md) - Complete guide
+- [Component Template](../templates/component-template.tsx) - Boilerplate
+- [Radix UI Docs](https://www.radix-ui.com/primitives)