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/specs/rag-bm25-hybrid/spec.md

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+# rag-bm25-hybrid Specification
+
+## Purpose
+TBD - created by archiving change rag-bm25-hybrid-drop-sdk. Update Purpose after archive.
+## Requirements
+### Requirement: Knowledge vector backend SHALL be a single Milvus V2 store
+
+Knowledge indexing and retrieval SHALL use one MilvusClientV2-backed store. Legacy MilvusServiceClient SDK search modes (`sdk`, `auto` fallback to SDK) SHALL NOT be used for `lookup_knowledge`.
+
+#### Scenario: No SDK search mode
+
+- **WHEN** knowledge retrieval executes
+- **THEN** it SHALL NOT call legacy SDK `search` APIs for candidate generation
+
+### Requirement: Hybrid mode SHALL fuse dense ANN and BM25 sparse ANN
+
+When search mode is hybrid, the store SHALL query dense vectors and BM25 sparse vectors and fuse results with RRF (or equivalent ranker) before returning hits.
+
+#### Scenario: Hybrid uses BM25 text query
+
+- **WHEN** hybrid search runs with a text query
+- **THEN** one search leg SHALL use the BM25/sparse field with the raw query text
+- **AND** another leg SHALL use the dense embedding of the query
+
+### Requirement: Write path SHALL populate BM25 input text and dense vectors
+
+Document chunk indexing SHALL write original content, BM25 input text, dense embedding, and metadata required for chunk identity.
+
+#### Scenario: Index writes search_text and vector
+
+- **WHEN** a chunk is indexed
+- **THEN** the store row SHALL include searchable text for BM25 and a dense vector field
+