feat(rag): dense+BM25 hybrid on MilvusClientV2, drop SDK path

Replace legacy MilvusServiceClient knowledge search/write with a single
MilvusClientV2 hybrid store (BM25 function + dense ANN + RRFRanker).
Use collection biz_hybrid and require knowledge reindex.

devflow/projects/2026-07-27-rag-bm25-hybrid-drop-sdk/acceptance.md

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+# Acceptance
+
+## Done
+
+- `MilvusHybridKnowledgeStore`: schema BM25 function + dense, hybridSearch+RRFRanker, dense search
+- `VectorSearchService` only routes dense|hybrid to V2 store
+- `VectorIndexService` writes via V2 store
+- Removed knowledge-path `MilvusServiceClient` bean wiring
+- Config: `milvus.collection=biz_hybrid`, `retrieval.search.mode=hybrid`
+
+## Tests
+
+```text
+mvn -Dtest=LookupKnowledgeToolTest,KnowledgeEvidencePostProcessorTest,RagResultProjectorTest,RrfFusionTest,VectorKnowledgeSearchAdapterHybridTest,VectorSearchServiceTest,VectorIndexServiceTest test
+```
+
+EXIT:0
+
+## Ops note
+
+Reindex all knowledge docs into `biz_hybrid` before production hybrid search is meaningful.
+Legacy `biz` collection is unused by knowledge path.

devflow/projects/2026-07-27-rag-bm25-hybrid-drop-sdk/brief.md

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+# Brief: rag-bm25-hybrid-drop-sdk
+
+True dense+BM25 hybrid on a single MilvusClientV2 backend. Legacy SDK search/write for knowledge path removed. New collection `biz_hybrid` requires reindex.

devflow/projects/2026-07-27-rag-bm25-hybrid-drop-sdk/decisions.md

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+# Decisions
+
+- Single backend: MilvusClientV2 only for knowledge RAG
+- Drop sdk/spring/auto retrieval routing
+- New collection biz_hybrid to avoid mutating legacy biz schema in place
+- Hybrid = dense ANN + BM25 sparse ANN + RRFRanker
+- Dense L2 enrichment for threshold compatibility on hybrid hits