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/continuous-learning/README.md

@@ -0,0 +1,11 @@
+# 持续学习
+
+持续学习定义 GoodBuddy 如何从运行结果、评估证据和人工反馈中形成可验证的改进闭环。
+
+## 文档导航
+
+| 文档 | 权威职责 |
+| --- | --- |
+| [持续学习与评估门 PRD](./prd.md) | 产品目标、学习边界、评估门和验收要求 |
+
+当前尚无独立 User Stories、功能逻辑、UI、技术设计和进度文档。