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/next-best-practices/async-patterns.md

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+# Async Patterns
+
+In Next.js 15+, `params`, `searchParams`, `cookies()`, and `headers()` are asynchronous.
+
+## Async Params and SearchParams
+
+Always type them as `Promise<...>` and await them.
+
+### Pages and Layouts
+
+```tsx
+type Props = { params: Promise<{ slug: string }> }
+
+export default async function Page({ params }: Props) {
+  const { slug } = await params
+}
+```
+
+### Route Handlers
+
+```tsx
+export async function GET(
+  request: Request,
+  { params }: { params: Promise<{ id: string }> }
+) {
+  const { id } = await params
+}
+```
+
+### SearchParams
+
+```tsx
+type Props = {
+  params: Promise<{ slug: string }>
+  searchParams: Promise<{ query?: string }>
+}
+
+export default async function Page({ params, searchParams }: Props) {
+  const { slug } = await params
+  const { query } = await searchParams
+}
+```
+
+### Synchronous Components
+
+Use `React.use()` for non-async components:
+
+```tsx
+import { use } from 'react'
+
+type Props = { params: Promise<{ slug: string }> }
+
+export default function Page({ params }: Props) {
+  const { slug } = use(params)
+}
+```
+
+### generateMetadata
+
+```tsx
+type Props = { params: Promise<{ slug: string }> }
+
+export async function generateMetadata({ params }: Props): Promise<Metadata> {
+  const { slug } = await params
+  return { title: slug }
+}
+```
+
+## Async Cookies and Headers
+
+```tsx
+import { cookies, headers } from 'next/headers'
+
+export default async function Page() {
+  const cookieStore = await cookies()
+  const headersList = await headers()
+
+  const theme = cookieStore.get('theme')
+  const userAgent = headersList.get('user-agent')
+}
+```
+
+## Migration Codemod
+
+```bash
+npx @next/codemod@latest next-async-request-api .
+```