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.

src/prefs-shared.ts

@@ -33,6 +33,14 @@ export interface SidebarPrefs {
    * user explicitly enables it in the side card settings.
    */
   agentTerminalTools: boolean
+  /**
+   * Whether the model-facing `sidebar_open` tool is injected into the
+   * model's toolset — one tool that lets the model actively open a local
+   * file, a local folder (as a tree rooted there), or an HTTP(S) page in
+   * the calling session's sidebar. Off by default: the feature stays
+   * dormant until the user explicitly enables it in the side card settings.
+   */
+  agentOpenTools: boolean
   /**
    * Custom terminal font-family stack (a CSS font-family value, e.g.
    * `'JetBrains Mono', monospace`). Empty string follows the app's theme
@@ -229,6 +237,7 @@ export const SIDEBAR_PREFS_DEFAULTS: SidebarPrefs = {
   autoOpenSubagent: true,
   autoOpenJobs: true,
   agentTerminalTools: false,
+  agentOpenTools: false,
   bottomPanelAutoTerminal: true,
   terminalFontFamily: '',
   terminalFontSize: TERMINAL_FONT_SIZE_DEFAULT,