docs: organize product design by feature

Define repository and feature-scoped documentation rules for PRDs, User Stories, logic, UI, technical design, and progress evidence.

Move feature-owned documents under docs/features, add one entry point per feature, remove the redundant docs index, and update all affected cross-references. Expand the local tool environment design with explicit product, story, logic, UI, technical, and progress documents.

docs/features/memory/README.md

@@ -0,0 +1,11 @@
+# 记忆
+
+记忆定义 GoodBuddy 的用户、项目和会话记忆分区、写入边界、召回和治理。
+
+## 文档导航
+
+| 文档 | 权威职责 |
+| --- | --- |
+| [分区记忆 PRD](./prd.md) | 记忆范围、分区、生命周期和产品验收 |
+
+当前尚无独立 User Stories、功能逻辑、UI、技术设计和进度文档。