feat: sidebar_open tool gates model-driven sidebar opens behind a global setting

New global Side card setting `agentOpenTools` (default off, 常规 group):
when enabled, one `sidebar_open` tool is injected into the model so it can
actively open a local file, a local folder (tree rooted there), or an
HTTP(S) page in the calling session's sidebar.

- host: AgentOpenRegistry + registerOpenTool (src/agent-opens.ts); the tool
  classifies http(s) URLs / stat-driven file/folder paths, refuses other
  schemes, resolves relative paths against the session cwd, and reports
  target-tab-disabled instead of silently no-oping. Gated like
  agentTerminalTools (register/drain on settings watch).
- delivery: /sidebar/ws/agent-opens push (consume-on-send queue, replay on
  attach); the tool result reports `delivered` so queued opens for
  non-active sessions are honest.
- client: Sidebar subscribes and routes pushes to editor (file), folder
  window (meta.dir, tree rooted at the folder — EditorHost), or browser
  (URL) tabs; settings row in 常规 + 20 locale dictionaries.
- tests: agent-opens registry/tool spec, smoke gating + WS route list,
  prefs/plugin-shape defaults, editor-host folder window, side-card rows.
- docs: design doc, AGENTS.md key lists + reference, README feature bullets.

tests/editor-host.spec.tsx

@@ -293,4 +293,31 @@ describe('EditorHost (files window)', () => {
       unmount()
     }
   })
+
+  it('a folder tab (meta.dir) renders the tree rooted at the folder, no editor chrome', () => {
+    const { store, ctx } = setup()
+    ctx.betterSidebar!.openTab({
+      type: 'editor',
+      title: 'src',
+      path: '/work/src',
+      id: 'editor:/work/src',
+      meta: { dir: true },
+    }, { sessionId: 'editor-home-session' })
+    const dirTab = (): SidebarTab =>
+      allLeaves(store.getSnapshot().state!.splits).flatMap(leaf => leaf.tabs)
+        .find(tab => tab.path === '/work/src')!
+    const { container, unmount } = mountHost(ctx, store, dirTab)
+    try {
+      const html = container.innerHTML
+      // The folder window is the full tree surface: the folder basename is
+      // the tree root row and the search box is present; the editor empty
+      // hint and the file path input are NOT.
+      expect(html).toContain('src')
+      expect(html).toContain('Search files by name…')
+      expect(html).not.toContain('Pick a file from the tree panel')
+      expect(html).not.toContain('File path (relative')
+    } finally {
+      unmount()
+    }
+  })
 })