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/client/service.ts

@@ -117,8 +117,8 @@ export interface SidebarSettingsDeclaration {
    * Extra settings rows rendered under the feature's own row in the
    * settings page (only while the feature is enabled). Keys must be fields
    * of the host's PrefsSchema (built-ins: 'autoOpenSubagent',
-   * 'agentTerminalTools', 'terminalFontFamily'); unknown keys are dropped
-   * by the settings seam.
+   * 'agentTerminalTools', 'agentOpenTools', 'terminalFontFamily'); unknown
+   * keys are dropped by the settings seam.
    */
   toggles?: readonly SidebarSettingToggle[]
   /**