feat: add dynamic Sugar docs fetcher and /refresh-docs endpoint (fixes #91)

[skypank-coder]

Summary

Adds a dynamic document fetching system to keep the RAG
vectorstore current with the latest Sugar activity docs.

Problem

The RAG agent had no Sugar activity creation docs — questions
like "How do I create a Sugar activity?" returned poor answers.
Also, there was no way to update docs without restarting the server.

Changes

• scripts/fetch_sugar_docs.py — fetches live Sugar docs
  from GitHub API (desktop-activity, web-activity, contributing)
• POST /refresh-docs — admin endpoint to re-fetch docs and
  rebuild FAISS vectorstore at runtime, zero downtime
• app/ai.py — UTF-8 encoding fix
• app/routes/api.py — refresh-docs route added
• .example.env — added GITHUB_TOKEN variable
• README.md — added doc refresh documentation
• requirements.txt — added requests library

Testing

✅ All 3 Sugar docs fetch successfully (~35KB)
✅ /refresh-docs rebuilds vectorstore correctly
✅ /ask?question=How do I create a Sugar activity?
now returns accurate Sugar activity guidance
✅ All existing endpoints unchanged

Fixes #91