docs(agents): slim AGENTS.md to repo-global rules, single-source the API reference

AGENTS.md (546 -> 63 lines) now carries only project-global development
rules: hard constraints, mount-smoke CI, DSH 0.1.2-alpha.1 adaptation
notes, npm release, a quick-rule list, and a docs/tests map. The
consumer plugin API reference lives solely in
docs/external-plugin-guide.md.

Reference sync:
- README/README_EN: collapse the dual 'full integration docs' links to
  the guide; AGENTS.md stays linked as the repo rules
- shell-presets.ts / mount.e2e.ts comments: skinning contract pointer
  moved from AGENTS.md §8 to guide §12

README.md

@@ -449,9 +449,7 @@ GitHub topic [`dsh-better-sidebar`](https://github.com/topics/dsh-better-sidebar
 
 从 v0.4.0 起暴露 `ctx.betterSidebar` 服务，其他插件可注册侧边栏页面与文件预览器（内置 7 tab + 6 viewer 亦通过同一服务注册）。v0.12.1 补齐基座能力（完整类型导出、能力探测、状态订阅、tab 角标、生命周期回调、定向打开、插件自有设置等）。
 
-完整接入文档：
-- **[`AGENTS.md`](./AGENTS.md)**——仓库内维护的接入文档（全字段、匹配算法、HMR 陷阱、声明式设置、版本探测）；
-- **[`docs/external-plugin-guide.md`](./docs/external-plugin-guide.md)**——面向外部插件开发者的接入指南（含完整最小示例）。
+完整接入文档（全字段、匹配算法、HMR 陷阱、声明式设置、版本探测、自由窗口与皮肤契约）：**[`docs/external-plugin-guide.md`](./docs/external-plugin-guide.md)**；仓库开发规则（硬约束 / CI / 发版）见 [`AGENTS.md`](./AGENTS.md)。
 
 ### ➕ 添加插件（推荐插件目录）