- 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- add py-rag chain exploration note; mark superseded Milvus/L0 notes with status banners
- close ISS-017 (superseded by L0 sinking); mark knowledge_domain table orphaned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
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.
zhuyongxin