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/internal-comms-anthropic/examples/general-comms.md

@@ -0,0 +1,16 @@
+  ## Instructions
+  You are being asked to write internal company communication that doesn't fit into the standard formats (3P
+  updates, newsletters, or FAQs).
+
+  Before proceeding:
+  1. Ask the user about their target audience
+  2. Understand the communication's purpose
+  3. Clarify the desired tone (formal, casual, urgent, informational)
+  4. Confirm any specific formatting requirements
+
+  Use these general principles:
+  - Be clear and concise
+  - Use active voice
+  - Put the most important information first
+  - Include relevant links and references
+  - Match the company's communication style