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/index.tsx

@@ -12,7 +12,7 @@ import { createElement } from 'react'
 import { createRoot, type Root } from 'react-dom/client'
 import type { Context } from '../context-types.ts'
 import { createSidebarStore } from './state.ts'
-import { createBetterSidebarService } from './service.ts'
+import { createBetterSidebarService, matchUrlTarget } from './service.ts'
 import { resetChunks } from './chunk-loader.ts'
 import { registerBuiltins } from './builtins/index.ts'
 import { Sidebar } from './Sidebar.tsx'
@@ -153,16 +153,35 @@ export function apply(ctx: Context): void {
     ctx.effect(
       () => {
         try {
-          // External http(s) links in the chat/GUI open the sidebar browser
-          // instead of a new window (gated on the browserInterceptLinks pref
-          // AND the browser tab's enable switch; Ctrl/Cmd+click bypasses).
+          // External http(s) links in the chat/GUI open the sidebar instead
+          // of a new window. Gated on the browserInterceptLinks MASTER pref,
+          // the URL's protocol flag (browserInterceptHttp / Https — https
+          // defaults OFF: most https sites refuse iframe embedding), and the
+          // target tab's enable switch; Ctrl/Cmd+click always bypasses. The
+          // target is the first registered tab whose `urlTarget` claims the
+          // URL (enabled tabs only), else the built-in browser tab.
+          const urlTargetOf = (url: URL): string | undefined => {
+            const prefs = sidebarStore.getPrefs()
+            const enabled = service.getTabs().filter(tab => prefs.tabsEnabled[tab.id] !== false)
+            return matchUrlTarget(enabled, url)?.id
+          }
           return registerLinkInterception({
-            takeoverEnabled: () => sidebarStore.getPrefs().browserInterceptLinks !== false
-              && sidebarStore.getPrefs().tabsEnabled['browser'] !== false,
+            takeoverEnabled: (url) => {
+              const prefs = sidebarStore.getPrefs()
+              if (prefs.browserInterceptLinks === false) return false
+              const protocolOn = url.protocol === 'https:'
+                ? prefs.browserInterceptHttps !== false
+                : prefs.browserInterceptHttp !== false
+              if (!protocolOn) return false
+              // A plugin claim is the target (already enabled-filtered);
+              // otherwise the built-in browser must be enabled.
+              return urlTargetOf(url) !== undefined || prefs.tabsEnabled['browser'] !== false
+            },
             openInSidebar: (url) => {
               let title: string | undefined
               try { title = new URL(url).hostname } catch { /* keep the default title */ }
-              ctx.betterSidebar?.openTab({ type: 'browser', url, title })
+              const type = urlTargetOf(new URL(url)) ?? 'browser'
+              ctx.betterSidebar?.openTab({ type, url, title })
             },
             selfOrigin: window.location.origin,
           })