docs(mvp): move engineering notes under mvp/engineering

Relocate RAG and diagnosis decision/E2E writeups from docs/ root into
mvp/engineering so architecture, issues, and engineering narrative stay
together. Update indexes and cross-links; leave docs/learning as legacy.

eval/rag-retrieval/README.md

@@ -9,8 +9,8 @@ Production knowledge path: `MilvusHybridKnowledgeStore` with `retrieval.search.m
 
 Related design notes:
 
-- `docs/RAG-Hybrid质量分与后处理.md`
-- `docs/RAG-Agent如何读relevance_level.md`
+- `mvp/engineering/rag/RAG-Hybrid质量分与后处理.md`
+- `mvp/engineering/rag/RAG-Agent如何读relevance_level.md`
 - `mvp/architecture/RAG知识检索架构.md` §6
 
 ## Offline vs live