feat: add 33 skills from vudovn/antigravity-kit

- Added: api-patterns, app-builder, architecture, bash-linux, behavioral-modes,
  clean-code, code-review-checklist, database-design, deployment-procedures,
  docker-expert, documentation-templates, game-development, geo-fundamentals,
  i18n-localization, lint-and-validate, mobile-design, nestjs-expert,
  nextjs-best-practices, nodejs-best-practices, parallel-agents,
  performance-profiling, plan-writing, powershell-windows, prisma-expert,
  python-patterns, react-patterns, red-team-tactics, seo-fundamentals,
  server-management, tailwind-patterns, tdd-workflow, typescript-expert,
  vulnerability-scanner
- Updated README: skill count 179 → 223
- Added credit for vudovn/antigravity-kit (MIT License)

Source: https://github.com/vudovn/antigravity-kit

skills/app-builder/templates/nextjs-saas/TEMPLATE.md

@@ -0,0 +1,100 @@
+---
+name: nextjs-saas
+description: Next.js SaaS template principles. Auth, payments, email.
+---
+
+# Next.js SaaS Template
+
+## Tech Stack
+
+| Component | Technology |
+|-----------|------------|
+| Framework | Next.js 14 (App Router) |
+| Auth | NextAuth.js v5 |
+| Payments | Stripe |
+| Database | PostgreSQL + Prisma |
+| Email | Resend |
+| UI | Tailwind (ASK USER: shadcn/Headless UI/Custom?) |
+
+---
+
+## Directory Structure
+
+```
+project-name/
+├── prisma/
+├── src/
+│   ├── app/
+│   │   ├── (auth)/      # Login, register
+│   │   ├── (dashboard)/ # Protected routes
+│   │   ├── (marketing)/ # Landing, pricing
+│   │   └── api/
+│   │       ├── auth/[...nextauth]/
+│   │       └── webhooks/stripe/
+│   ├── components/
+│   │   ├── auth/
+│   │   ├── billing/
+│   │   └── dashboard/
+│   ├── lib/
+│   │   ├── auth.ts      # NextAuth config
+│   │   ├── stripe.ts    # Stripe client
+│   │   └── email.ts     # Resend client
+│   └── config/
+│       └── subscriptions.ts
+└── package.json
+```
+
+---
+
+## SaaS Features
+
+| Feature | Implementation |
+|---------|---------------|
+| Auth | NextAuth + OAuth |
+| Subscriptions | Stripe Checkout |
+| Billing Portal | Stripe Portal |
+| Webhooks | Stripe events |
+| Email | Transactional via Resend |
+
+---
+
+## Database Schema
+
+| Model | Fields |
+|-------|--------|
+| User | id, email, stripeCustomerId, subscriptionId |
+| Account | OAuth provider data |
+| Session | User sessions |
+
+---
+
+## Environment Variables
+
+| Variable | Purpose |
+|----------|---------|
+| DATABASE_URL | Prisma |
+| NEXTAUTH_SECRET | Auth |
+| STRIPE_SECRET_KEY | Payments |
+| STRIPE_WEBHOOK_SECRET | Webhooks |
+| RESEND_API_KEY | Email |
+
+---
+
+## Setup Steps
+
+1. `npx create-next-app {{name}} --typescript --tailwind --app`
+2. Install: `npm install next-auth @auth/prisma-adapter stripe resend`
+3. Setup Stripe products/prices
+4. Configure environment
+5. `npm run db:push`
+6. `npm run stripe:listen` (webhooks)
+7. `npm run dev`
+
+---
+
+## Best Practices
+
+- Route groups for layout separation
+- Stripe webhooks for subscription sync
+- NextAuth with Prisma adapter
+- Email templates with React Email