feat(rag): minimal BM25 retriever and /tools/retrieve

[haz3141]

Implements Step 5 minimal RAG pipeline with:

• Ingestion: - loads text files from data directory
• Chunking: - simple sentence-based chunking with configurable max chars
• Retrieval: - BM25 retriever using rank-bm25 library
• MCP Endpoint: - exposes RAG functionality via HTTP API
• Tests: Deterministic unit tests with no external dependencies
• CI: Updated workflow to run RAG tests alongside existing tests
• Config: Environment variables for data directory and top-k results

Dependencies: Only adds - lightweight and deterministic
Testing: Uses repo fixtures, no network calls, fully deterministic
Configuration: and environment variables