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/async-parallel.md

@@ -0,0 +1,28 @@
+---
+title: Promise.all() for Independent Operations
+impact: CRITICAL
+impactDescription: 2-10× improvement
+tags: async, parallelization, promises, waterfalls
+---
+
+## Promise.all() for Independent Operations
+
+When async operations have no interdependencies, execute them concurrently using `Promise.all()`.
+
+**Incorrect (sequential execution, 3 round trips):**
+
+```typescript
+const user = await fetchUser()
+const posts = await fetchPosts()
+const comments = await fetchComments()
+```
+
+**Correct (parallel execution, 1 round trip):**
+
+```typescript
+const [user, posts, comments] = await Promise.all([
+  fetchUser(),
+  fetchPosts(),
+  fetchComments()
+])
+```