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/full-duplex-voice/README.md

@@ -0,0 +1,11 @@
+# 全双工语音
+
+全双工语音定义持续听说、打断、语音活动检测、本地与远端推理以及多平台音频边界。
+
+## 文档导航
+
+| 文档 | 权威职责 |
+| --- | --- |
+| [技术设计](./technical-design.md) | 产品目标、交互流程、音频架构、模型、打包和实施状态 |
+
+现有文档是综合设计，尚未按 PRD、User Stories、功能逻辑、UI 和进度职责拆分。