feat: Add 5 curated skills for Mosaic Stack

New skills:
- next-best-practices: Next.js 15+ RSC, async patterns, self-hosting (vercel-labs)
- better-auth-best-practices: Official Better-Auth with Drizzle adapter (better-auth)
- verification-before-completion: Evidence-based completion claims (obra/superpowers)
- shadcn-ui: Component patterns with Tailwind v4 adaptation note (developer-kit)
- writing-skills: TDD methodology for skill authoring (obra/superpowers)

README reorganized by category with Mosaic Stack alignment section.
Total: 9 skills (4 existing + 5 new).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

skills/shadcn-ui/references/learn.md

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+# shadcn/ui Learning Guide
+
+This guide helps you learn shadcn/ui from basics to advanced patterns.
+
+## Learning Path
+
+### 1. Understanding the Philosophy
+
+shadcn/ui is different from traditional component libraries:
+
+- **Copy-paste components**: Components are copied into your project, not installed as packages
+- **Full customization**: You own the code and can modify it freely
+- **Built on Radix UI**: Provides accessibility primitives
+- **Styled with Tailwind**: Uses utility classes for consistent styling
+
+### 2. Core Concepts to Master
+
+#### Class Variance Authority (CVA)
+Most components use CVA for variant management:
+
+```tsx
+const buttonVariants = cva(
+  "base-classes",
+  {
+    variants: {
+      variant: {
+        default: "variant-classes",
+        destructive: "destructive-classes",
+      },
+      size: {
+        default: "size-classes",
+        sm: "small-classes",
+      },
+    },
+    defaultVariants: {
+      variant: "default",
+      size: "default",
+    },
+  }
+)
+```
+
+#### cn Utility Function
+The `cn` function combines classes and resolves conflicts:
+
+```tsx
+import { clsx, type ClassValue } from "clsx"
+import { twMerge } from "tailwind-merge"
+
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs))
+}
+```
+
+### 3. Installation Checklist
+
+- [ ] Initialize a new project (Next.js, Vite, or Remix)
+- [ ] Install Tailwind CSS
+- [ ] Run `npx shadcn@latest init`
+- [ ] Configure CSS variables
+- [ ] Install first component: `npx shadcn@latest add button`
+
+### 4. Essential Components to Learn First
+
+1. **Button** - Learn variants and sizes
+2. **Input** - Form inputs with labels
+3. **Card** - Container components
+4. **Form** - Form handling with React Hook Form
+5. **Dialog** - Modal windows
+6. **Select** - Dropdown selections
+7. **Toast** - Notifications
+
+### 5. Common Patterns
+
+#### Form Pattern
+Every form follows this structure:
+
+```tsx
+1. Define Zod schema
+2. Create form with useForm
+3. Wrap with Form component
+4. Add FormField for each input
+5. Handle submission
+```
+
+#### Component Customization Pattern
+To customize a component:
+
+1. Copy component to your project
+2. Modify the variants
+3. Add new props if needed
+4. Update types
+
+### 6. Best Practices
+
+- Always use TypeScript
+- Follow the existing component structure
+- Use semantic HTML when possible
+- Test with screen readers for accessibility
+- Keep components small and focused
+
+### 7. Advanced Topics
+
+- Creating custom components from scratch
+- Building complex forms with validation
+- Implementing dark mode
+- Optimizing for performance
+- Testing components
+
+## Practice Exercises
+
+### Exercise 1: Basic Setup
+1. Create a new Next.js project
+2. Set up shadcn/ui
+3. Install and customize a Button component
+4. Add a new variant "gradient"
+
+### Exercise 2: Form Building
+1. Create a contact form with:
+   - Name input (required)
+   - Email input (email validation)
+   - Message textarea (min length)
+   - Submit button with loading state
+
+### Exercise 3: Component Combination
+1. Build a settings page using:
+   - Card for layout
+   - Sheet for mobile menu
+   - Select for dropdowns
+   - Switch for toggles
+   - Toast for notifications
+
+### Exercise 4: Custom Component
+1. Create a custom Badge component
+2. Support variants: default, secondary, destructive, outline
+3. Support sizes: sm, default, lg
+4. Add icon support
+
+## Resources
+
+- [Official Documentation](https://ui.shadcn.com)
+- [GitHub Repository](https://github.com/shadcn/ui)
+- [Examples Gallery](https://ui.shadcn.com/examples)
+- [Radix UI Primitives](https://www.radix-ui.com/primitives)
+- [Tailwind CSS Documentation](https://tailwindcss.com/docs)