SmartRetrieval RRF+MMR

Single-strategy retrieval (BM25 or vector-only) misses complementary signals in the ReasoningBank.

Combine BM25 and TF-IDF scores via Reciprocal Rank Fusion, then apply Max Marginal Relevance to reduce redundancy.

Improved retrieval quality on internal evals. ~5ms overhead measured on 200 trajectories. 26 tests cover the pipeline end-to-end.