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.

mvp/README.md

@@ -1,60 +1,41 @@
 # SuperBizAgent MVP 文档
 
-**更新日期**：2026-07-23
+**更新日期**：2026-07-29
 
-本目录保存 MVP 阶段的架构、问题、演示、评测和数据表说明。当前材料按“当前入口”和“历史归档”拆开，避免把早期设计稿当成当前实现。
+本目录保存 MVP 阶段的架构、工程纪要、问题、演示、评测和数据表说明。当前材料按“当前入口”和“历史归档”拆开，避免把早期设计稿当成当前实现。
 
 ## 当前入口
 
 | 目录/文档 | 用途 |
 |---|---|
-| [architecture/README.md](architecture/README.md) | 当前 MVP 架构入口 |
+| [architecture/README.md](architecture/README.md) | **现行架构**（系统现在怎么跑） |
+| [engineering/README.md](engineering/README.md) | **工程纪要**（问题 / 决策 / 思路 / E2E 导读） |
 | [architecture/current-mvp-architecture.md](architecture/current-mvp-architecture.md) | 当前可运行系统架构 |
 | [architecture/agent-orchestration.md](architecture/agent-orchestration.md) | Agent 编排架构 |
 | [architecture/harness-quality-gates.md](architecture/harness-quality-gates.md) | Harness 与质量门禁 |
 | [architecture/session-trace-lifecycle.md](architecture/session-trace-lifecycle.md) | 会话与 Trace 生命周期 |
+| [architecture/RAG知识检索架构.md](architecture/RAG知识检索架构.md) | 当前 hybrid 检索架构 |
 | [issues/README.md](issues/README.md) | MVP issue 索引 |
 | [tables/README.md](tables/README.md) | 当前 MySQL 表说明 |
 | [demo/README.md](demo/README.md) | Demo 运行和演示材料 |
 | [eval/README.md](eval/README.md) | 诊断评测材料 |
 
-当前架构、运行链路和后续规划分别以 `architecture/`、`issues/README.md`、`tables/README.md` 以及 OpenSpec/devflow 的最新记录为准。
+**读法**：改行为先看 `architecture/`；要理解「为什么这样定、踩过什么坑」再看 `engineering/`。  
+早期个人学习笔记仍在仓库根目录 `docs/learning/` 等，**可能过时**，不以之为现行口径。
 
 ## 文档结构
 
 ```text
 mvp/
-  architecture/
+  architecture/          # 现行架构规范
+  engineering/           # 工程纪要（问题/决策/E2E）
     README.md
-    current-mvp-architecture.md
-    agent-orchestration.md
-    harness-quality-gates.md
-    session-trace-lifecycle.md
-    archive/
-      2026-07-05-legacy/
-      2026-07-22-legacy/
-  issues/
-    README.md
-    active/
-    archived/
-    design-notes/
     rag/
+    diagnosis/
+  issues/
   tables/
-    README.md
-    *表-*.md
-    archive/
   demo/
-    README.md
-    ten-minute-interview-demo.md
-    requests/
-    scripts/
-    output/
   eval/
-    README.md
-    schema.md
-    cases/
-    fixtures/
-    reports/
   archive/
 ```