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.

openspec/changes/archive/2026-07-27-rag-bm25-hybrid-drop-sdk/proposal.md

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+# Change: Real BM25 hybrid search and drop legacy SDK path
+
+## Why
+
+Delivery 2 shipped application-layer multi-path + sparse-lite lexical ranking. Product requirement is **true dense + BM25 hybrid retrieval**. Legacy `MilvusServiceClient` search path will be abandoned in this change; keep a single knowledge vector backend based on Milvus Java SDK v2 (`MilvusClientV2`) with BM25 function + `hybridSearch` + RRF.
+
+## What Changes
+
+- New hybrid collection schema: dense float vector + analyzed text + sparse BM25 output
+- Write path inserts text for BM25 auto-sparsification and dense embedding
+- Search path: single backend
+  - dense mode: dense ANN only
+  - hybrid mode: dense ANN + BM25 sparse ANN fused by RRFRanker
+- Remove/disable legacy SDK `search` / `searchSimilarDocumentsWithSdk` knowledge path and mode routing (`sdk|spring|auto`)
+- KnowledgeSearchPort adapter calls the single backend
+- Config: collection name, search mode, rrf-k, path weights optional
+- Tests for store mapping/fusion wiring; no dependency on live Zilliz in unit tests
+
+## Non-goals
+
+- Automatic full production reindex job UI
+- Cross-encoder model rerank
+- Keeping dual Spring-AI + SDK knowledge search modes
+
+## Impact
+
+- **Breaking for existing `biz` collection**: requires new hybrid collection + reindex
+- Agent tool contract unchanged
+- Interface: L2 internal retrieval backend
+
+## Depends on
+
+- Delivery 1 chunk identity
+- Delivery 2 SearchPort (will replace sparse-lite hybrid implementation)