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.

openspec/changes/archive/2026-07-27-rag-chunk-evidence-identity-dedup/proposal.md

@@ -9,7 +9,7 @@
 
 As a result, L1 can recall multiple useful chunks from one document, but Agent often sees only one. This blocks multi-path / hybrid retrieval benefits and hurts long runbook completeness.
 
-This change is **Delivery 1** from `docs/Milvus-Hybrid接入清单.md`. Hybrid schema/search is **out of scope** and will be a separate change after this is archived.
+This change is **Delivery 1** from `mvp/engineering/rag/Milvus-Hybrid接入清单.md`. Hybrid schema/search is **out of scope** and will be a separate change after this is archived.
 
 ## What Changes
 
@@ -47,7 +47,7 @@ This change is **Delivery 1** from `docs/Milvus-Hybrid接入清单.md`. Hybrid s
 - Code: retrieval DTO/services, post-processor, lookup tool config, projector, tests
 - Agent-visible: more evidence items possible for same logical document when multiple chunks are relevant
 - Interface level: **L2/L3** — Agent `document_id` semantics become chunk-scoped evidence id (often `docId#chunk-N`); EvidenceGuard still validates against tool projection ids
-- Docs baseline: `docs/Milvus-Hybrid接入清单.md` §1.1 Delivery 1
+- Docs baseline: `mvp/engineering/rag/Milvus-Hybrid接入清单.md` §1.1 Delivery 1
 
 ## Risks