feat: 服务化 API 新增 urlTarget 外链认领 + 浏览器 HTTP/HTTPS 二级设置开关

基于 PR #74 的颗粒度讨论向前兼容拓展服务化 API：
- TabDescriptor 新增可选 urlTarget(url) 谓词：聊天/界面外链被拦截时按注册
  顺序路由到第一个命中的插件 tab 类型（openTab({ type, url, title })），
  内置 browser 退居隐式兜底；SIDEBAR_FEATURES 追加 'urlTarget'
- 浏览器链接设置改为二级菜单（齿轮弹窗）：新增 browserInterceptHttp
  （默认开）+ browserInterceptHttps（默认关）；browserInterceptLinks
  保留为总闸（默认开，旧文档零迁移）
- 拦截管线：registerLinkInterception 的 takeoverEnabled 改收解析后的
  URL，协议/同源纯判定（shouldInterceptLink）签名不变
- 测试：matchUrlTarget（注册顺序/抛错吞掉/browser 永不命中）、jsdom
  点击接管门控、parsePrefs 新字段默认值；全默认字面量同步
- 文档：设计文档 + AGENTS.md（字段/示例/设置键）+ README（外链分流）

README_EN.md

@@ -20,7 +20,7 @@
 ## ✨ Features
 
 - **🗂️ File Workbench**: file explorer (lazy-loading tree) + CodeMirror editor; inline preview for images / Markdown / HTML / PDF / Office
-- **🌐 Embedded Browser**: multiple web tabs with back / forward / refresh; content runs in a sandboxed iframe; external links open in the sidebar by default
+- **🌐 Embedded Browser**: multiple web tabs with back / forward / refresh; content runs in a sandboxed iframe; external links are routed by protocol by default — HTTP opens in the sidebar, HTTPS goes to the system browser (both adjustable in settings)
 - **💻 Real Terminal**: xterm.js + node-pty real shell, reconnect with transcript replay; optionally injects `terminal_*` tools for the model
 - **🌿 Git Panel**: real diff + VSCode-style diff tabs, history, right-click to stage / commit / revert
 - **🧩 Background Tasks**: agent topology + background tasks (exit codes / live output / force-kill)
@@ -38,7 +38,7 @@
 
 | Feature | Description | Screenshot |
 |---|---|---|
-| 🔌 Service API base | Complete type exports + `version`/`features` capability detection, state subscription (`getSnapshot`/`subscribeState`), tab `badge`, `onOpen`/`onActivate`/`onClose` lifecycle callbacks, `updateTab`/`activateTab`/`openFile`, targeted open, `meta` persisted across reloads, plugin-owned settings (`pluginToggles`/`render`) | <a href="https://github.com/user-attachments/assets/946f7028-4967-461e-a750-d1b5056b62d0"><img width="480" alt="Service API base screenshot" src="https://github.com/user-attachments/assets/946f7028-4967-461e-a750-d1b5056b62d0" /></a> |
+| 🔌 Service API base | Complete type exports + `version`/`features` capability detection, state subscription (`getSnapshot`/`subscribeState`), tab `badge`, `onOpen`/`onActivate`/`onClose` lifecycle callbacks, `updateTab`/`activateTab`/`openFile`, targeted open, `meta` persisted across reloads, plugin-owned settings (`pluginToggles`/`render`), external-link claim (`urlTarget`) | <a href="https://github.com/user-attachments/assets/946f7028-4967-461e-a750-d1b5056b62d0"><img width="480" alt="Service API base screenshot" src="https://github.com/user-attachments/assets/946f7028-4967-461e-a750-d1b5056b62d0" /></a> |
 | ➕ Add Plugins | Recommended plugin catalog in settings + one-click copy install command; built-in Office preview moved to the recommended plugin | <a href="https://github.com/user-attachments/assets/d4385b7e-aab4-425d-a5c4-2da5da81a34e"><img width="480" alt="Add Plugins screenshot" src="https://github.com/user-attachments/assets/d4385b7e-aab4-425d-a5c4-2da5da81a34e" /></a> |
 | 🖱️ Tab-bar scroll | Mouse-wheel horizontal scrolling on the tab bar | |
 | 🐛 Fixes | Remote access 403 (trust fence now uses `trustedHosts`), sidebar crash [#31](https://github.com/omdsh-dev/DSH-better-sidebar/issues/31), Windows HTML-preview drive-path | |