add keyword/BM25/hybrid retrievers with wrapper support

[elinawang626-design]

Summary

This PR adds lexical retrieval options for RAG item recall, including:

• keyword retriever
• bm25 retriever
• hybrid retriever (BM25 + vector via RRF fusion)

It also adds OpenAI wrapper support for default retriever configuration and per-call retrieval override.

Changes

Retrieval

• Add keyword retriever option for RAG item recall
• Add bm25 retriever implementation
• Add hybrid retriever using Reciprocal Rank Fusion (RRF) over vector + BM25 results
• Update retriever validation/options in settings

Wrapper support

• Support default retriever in OpenAI wrapper config
• Allow per-call method / retriever override for RAG retrieval

Tests

• Add BM25 retrieval tests
• Add keyword retrieval tests

Tooling / Docs

• Update Makefile / test commands as needed for src layout
• Update README usage/docs
• Update lockfile (uv.lock)

Why

This improves retrieval flexibility for different memory types and query styles:

• keyword: simple lexical matching
• bm25: stronger lexical ranking
• hybrid: combines semantic retrieval and lexical retrieval for better recall/robustness

Validation

• Added and ran tests for keyword and BM25 retrieval
• Verified branch diff vs main includes retrieval, wrapper, settings, tests, and docs changes

Notes

This branch is currently ahead of main and includes the full retrieval feature set in a single PR for easier review.