feat: pre-built index packs for popular frameworks

[rajkumarsakthivel]

Summary

Ship pre-built CCE indexes for popular open source repos so users can try CCE instantly without cloning or indexing.

Motivation

Currently, trying CCE requires: clone a repo → run cce init → wait for indexing. Pre-built packs skip all of this. A user could run one command and immediately search FastAPI, Django, or Express without having the source locally.

This also serves as a demo tool: "see CCE in action on a repo you already know."

Approach

1. Build indexes for 3-5 popular repos (FastAPI, Django, Express, chi)
2. Package as compressed archives (vector store + FTS + graph, likely 3-10 MB each)
3. Host on GitHub Releases or a CDN
4. Add cce demo fastapi command that downloads the pack and lets you run searches immediately
5. Include the benchmark queries so users can reproduce our published numbers instantly

Considerations

• Packs are tied to a specific commit of the target repo (document which one)
• Packs are tied to the embedding model version (BAAI/bge-small-en-v1.5)
• Need a versioning scheme so packs are invalidated when the embedding model changes
• Keep packs small (index only, no source code, respect licenses)

Inspiration

jCodeMunch ships pre-built symbol indexes (e.g., React 932 MB repo → 3 MB pack). Same concept adapted to CCE's chunk-based approach.