Add Real-World Project: RAG Over a GitHub Repo

[abderrahim-lectures]

Adds a new Real-World Projects lesson: Build a RAG App Over a GitHub Repo (backlog idea #16).

What's included

• docs/projects/github-repo-rag/index.md — full lesson in the house skeleton, building directly on the notes-based RAG project. Core upgrades over that sibling: chunking code at function/class boundaries vs. prose at paragraph boundaries, per-chunk line-range metadata, and a generation prompt that demands [path.py:start-end] citations you can open and verify.
• docs/projects/github-repo-rag/_category_.json
• examples/github-repo-rag/ — runnable 4-script tool (prepare_repo.py, build_index.py, retrieve.py, query.py), 6-provider LLM support in query.py, plus notebook.ipynb, README, pyproject/uv.lock/env template.
• Registration: src/data/projects.ts, docs/projects/index.mdx, src/pages/index.tsx (English only — i18n in a follow-up PR).

Verification

• Pipeline tested end-to-end locally: cloned the course repo and indexed docs/projects/ → 3233 chunks embedded locally, retrieval returns scores in [-1,1] with correct file/line citations.
• npm run typecheck passes; npm run build passes all 4 locales (the only github-repo-rag warnings are the pre-existing translated ai-agent anchor pattern shared with rag-notes).

Closes #270