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/api-patterns/rate-limiting.md

@@ -0,0 +1,31 @@
+# Rate Limiting Principles
+
+> Protect your API from abuse and overload.
+
+## Why Rate Limit
+
+```
+Protect against:
+├── Brute force attacks
+├── Resource exhaustion
+├── Cost overruns (if pay-per-use)
+└── Unfair usage
+```
+
+## Strategy Selection
+
+| Type | How | When |
+|------|-----|------|
+| **Token bucket** | Burst allowed, refills over time | Most APIs |
+| **Sliding window** | Smooth distribution | Strict limits |
+| **Fixed window** | Simple counters per window | Basic needs |
+
+## Response Headers
+
+```
+Include in headers:
+├── X-RateLimit-Limit (max requests)
+├── X-RateLimit-Remaining (requests left)
+├── X-RateLimit-Reset (when limit resets)
+└── Return 429 when exceeded
+```