Add benchmark for Express.js repo

[rajkumarsakthivel]

Run the CCE benchmark suite against Express and publish results.

Why: Express is JavaScript, testing that tree-sitter JS chunking and retrieval work well on a real-world Node.js project. Different language, different file structure from our current FastAPI benchmark.

Steps:

1. Run python benchmarks/run_benchmark.py --repo https://github.com/expressjs/express.git --source-dir lib
2. Write 15-20 realistic coding queries for Express (middleware, routing, error handling, etc.)
3. Save results to benchmarks/results/express.md
4. Note any retrieval quality differences vs Python repos

Expected output: A markdown file with per-query token savings, recall@10, and latency stats.