docs(mvp): update RAG docs for py-rag extraction

- rewrite RAG architecture doc for py-rag service contract mapping, ingest and rebuild ops
- refresh observability/trace doc for post-L0 single-attempt semantics
- add py-rag chain exploration note; mark superseded Milvus/L0 notes with status banners
- close ISS-017 (superseded by L0 sinking); mark knowledge_domain table orphaned
- refresh architecture/mvp/engineering indexes

mvp/engineering/rag/Milvus-Hybrid接入清单.md

@@ -1,5 +1,10 @@
 # Milvus Hybrid Search 接入对照清单
 
+> **已过时（2026-09-29）**：RAG 模块已抽离为独立 py-rag 知识服务，进程内 Milvus
+> （`MilvusHybridKnowledgeStore` / `VectorSearchService`）与本文描述的接入路径已整体删除。
+> 当前检索架构与契约映射见 [../../architecture/RAG知识检索架构.md](../../architecture/RAG知识检索架构.md)。
+> 本文仅作历史决策追溯。
+
 **日期**：2026-07-27  
 **前提**：旧 Milvus SDK 直连检索路径后续废弃，不作为长期实现基础  
 **目标**：在现有 `lookup_knowledge` pipeline 上接入 dense + sparse/BM25 混合检索，融合优先走服务端 RRF