Added hybrid search: BM25 + semantic + RRF fusion

[markgewhite]

Summary

• Added BM25Index module using rank_bm25 for keyword search
• Added custom reciprocal_rank_fusion() (~15 lines) to merge ranked result lists with deduplication
• Added HybridRetriever orchestrating BM25 (top 20) + semantic (top 20) → RRF → top 30
• BM25 index rebuilds from ChromaDB on startup with logged timing
• Replaced semantic-only search in on_message with hybrid retrieval

Closes #7

Test plan

• BM25Index: build, search, relevance, empty index, metadata (5 tests)
• RRF fusion: merge, deduplicate, top_n, single list, empty (5 tests)
• HybridRetriever: combines both retrievers, result limits (2 tests)
• Manual: ask questions, verify improved retrieval with keyword matches

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