refactor: remove deprecated retriever module

RecursiveRetriever was a failed approach, not carried into TRM5.
- delete app/retriever/ (empty placeholder)
- drop retriever + train blocks from config/default.yaml
- renumber fidelity checklist 13->12 items (drop #4, shift up)
- sync core-goal text, dir tree, module-interaction diagrams
  across CLAUDE.md, ARCHITECTURE.md, overview.md, README.md
- reference/ kept intact as historical code

app/tree/index.py

@@ -1,7 +1,7 @@
 """三层树索引核心数据结构。
 
 定义 Video-Tree-TRM 的三层树状索引结构，是所有后续模块
-（builder、retriever、harness、search）的基础依赖。
+（builder、harness、search）的基础依赖。
 
 数据结构层次::