feat: integrate official Anthropic and Vercel Labs skills

- Add 8 new skills (62 total, up from 58)
- Official Anthropic skills: docx, pdf, pptx, xlsx, brand-guidelines, internal-comms
- Vercel Labs skills: react-best-practices, web-design-guidelines
- Implement dual-versioning: -official/-anthropic and -community suffixes
- Update README with new skill registry and credits
- Regenerate skills_index.json (62 skills validated)
- Add comprehensive walkthrough.md

BREAKING CHANGE: Document skills (docx/pdf/pptx/xlsx) renamed with version suffixes

skills/react-best-practices/metadata.json

@@ -0,0 +1,15 @@
+{
+  "version": "0.1.0",
+  "organization": "Vercel Engineering",
+  "date": "January 2026",
+  "abstract": "Comprehensive performance optimization guide for React and Next.js applications, designed for AI agents and LLMs. Contains 40+ rules across 8 categories, prioritized by impact from critical (eliminating waterfalls, reducing bundle size) to incremental (advanced patterns). Each rule includes detailed explanations, real-world examples comparing incorrect vs. correct implementations, and specific impact metrics to guide automated refactoring and code generation.",
+  "references": [
+    "https://react.dev",
+    "https://nextjs.org",
+    "https://swr.vercel.app",
+    "https://github.com/shuding/better-all",
+    "https://github.com/isaacs/node-lru-cache",
+    "https://vercel.com/blog/how-we-optimized-package-imports-in-next-js",
+    "https://vercel.com/blog/how-we-made-the-vercel-dashboard-twice-as-fast"
+  ]
+}