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）

src/client/link-intercept.ts

@@ -1,10 +1,12 @@
 /**
  * Chat/GUI external-link interception: clicking an http(s) link that points
  * OUTSIDE the GUI (chat messages, tool rows, prose mentions) opens the
- * sidebar browser instead of a new browser tab. Gated by BOTH the
- * `browserInterceptLinks` pref and the browser tab's enable switch; a
- * Ctrl/Cmd/Shift/Alt-modified click always bypasses the takeover so the
- * user can still force a real browser tab.
+ * sidebar instead of a new browser tab. Gated by the caller through
+ * `takeoverEnabled(url)` — the `browserInterceptLinks` master, the URL's
+ * protocol flag (`browserInterceptHttp` / `browserInterceptHttps`) and the
+ * target tab's enable switch — and a Ctrl/Cmd/Shift/Alt-modified click
+ * always bypasses the takeover so the user can still force a real browser
+ * tab.
  *
  * Only the GUI's OWN document is watched — links inside the browser tab's
  * sandboxed iframe live in another document and never bubble here (and
@@ -13,7 +15,10 @@
 
 /** The pure decision: the URL to open in the sidebar, or null to let the
  *  click fall through. Extracted so the policy is unit-testable without a
- *  DOM. `anchorHref` must be the ABSOLUTE href (`<a>.href` already is). */
+ *  DOM. `anchorHref` must be the ABSOLUTE href (`<a>.href` already is).
+ *  The protocol/same-origin policy lives HERE; the prefs gates (master +
+ *  protocol flags + target enablement) live in the caller's
+ *  `takeoverEnabled(url)` callback. */
 export function shouldInterceptLink(anchorHref: string, selfOrigin: string): string | null {
   let url: URL
   try {
@@ -23,7 +28,7 @@ export function shouldInterceptLink(anchorHref: string, selfOrigin: string): str
   }
   if (url.protocol !== 'http:' && url.protocol !== 'https:') return null
   // Same-origin links are GUI-internal navigation (settings pages, tool
-  // docs) — never routed into the sidebar browser.
+  // docs) — never routed into the sidebar.
   try {
     if (url.origin === new URL(selfOrigin).origin) return null
   } catch {
@@ -39,11 +44,13 @@ export function isPlainLeftClick(event: { button: number; metaKey: boolean; ctrl
 
 /**
  * Register the document-level click capture that funnels external links
- * into the sidebar browser. Returns the disposer (HMR-safe).
+ * into the sidebar. Returns the disposer (HMR-safe).
  */
 export function registerLinkInterception(opts: {
-  takeoverEnabled: () => boolean
-  /** Open the sidebar browser tab at `url` (the caller resolves the session). */
+  /** Whether the takeover may happen for THIS url (the caller's prefs
+   *  gates: master switch, protocol flag, target enablement). */
+  takeoverEnabled: (url: URL) => boolean
+  /** Open the sidebar tab at `url` (the caller resolves the target type). */
   openInSidebar: (url: string) => void
   /** The GUI's own origin (window.location.origin at registration). */
   selfOrigin: string
@@ -51,13 +58,13 @@ export function registerLinkInterception(opts: {
   const onClick = (event: MouseEvent): void => {
     if (!isPlainLeftClick(event)) return
     if (event.defaultPrevented) return
-    if (!opts.takeoverEnabled()) return
     const target = event.target
     if (target === null || typeof (target as Element).closest !== 'function') return
     const anchor = (target as Element).closest('a[href]') as HTMLAnchorElement | null
     if (anchor === null) return
     const url = shouldInterceptLink(anchor.href, opts.selfOrigin)
     if (url === null) return
+    if (!opts.takeoverEnabled(new URL(url))) return
     event.preventDefault()
     opts.openInSidebar(url)
   }