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

research-wiki/designs/2026-07-07-tree-module-design.md

@@ -10,7 +10,7 @@ status: approved
 
 ## 1. 背景与动机
 
-TRM5 的三大模块（建树、训练 harness、新题构建）中，建树是一切的地基——搜索 Agent、训练循环、检索器全部依赖树结构。当前 `app/tree/` 目录为空，需要从 reference 代码和 TRM4 迁移建树能力。
+TRM5 的三大模块（建树、训练 harness、新题构建）中，建树是一切的地基——搜索 Agent、训练循环全部依赖树结构。当前 `app/tree/` 目录为空，需要从 reference 代码和 TRM4 迁移建树能力。
 
 ### 1.1 现状