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/rules/_sections.md

@@ -0,0 +1,46 @@
+# Sections
+
+This file defines all sections, their ordering, impact levels, and descriptions.
+The section ID (in parentheses) is the filename prefix used to group rules.
+
+---
+
+## 1. Eliminating Waterfalls (async)
+
+**Impact:** CRITICAL  
+**Description:** Waterfalls are the #1 performance killer. Each sequential await adds full network latency. Eliminating them yields the largest gains.
+
+## 2. Bundle Size Optimization (bundle)
+
+**Impact:** CRITICAL  
+**Description:** Reducing initial bundle size improves Time to Interactive and Largest Contentful Paint.
+
+## 3. Server-Side Performance (server)
+
+**Impact:** HIGH  
+**Description:** Optimizing server-side rendering and data fetching eliminates server-side waterfalls and reduces response times.
+
+## 4. Client-Side Data Fetching (client)
+
+**Impact:** MEDIUM-HIGH  
+**Description:** Automatic deduplication and efficient data fetching patterns reduce redundant network requests.
+
+## 5. Re-render Optimization (rerender)
+
+**Impact:** MEDIUM  
+**Description:** Reducing unnecessary re-renders minimizes wasted computation and improves UI responsiveness.
+
+## 6. Rendering Performance (rendering)
+
+**Impact:** MEDIUM  
+**Description:** Optimizing the rendering process reduces the work the browser needs to do.
+
+## 7. JavaScript Performance (js)
+
+**Impact:** LOW-MEDIUM  
+**Description:** Micro-optimizations for hot paths can add up to meaningful improvements.
+
+## 8. Advanced Patterns (advanced)
+
+**Impact:** LOW  
+**Description:** Advanced patterns for specific cases that require careful implementation.