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/SideCardSection.tsx

@@ -913,6 +913,17 @@ export function SideCardSection({ store, service }: SideCardSectionProps) {
             onChange={(next) => { applyPref({ interceptOpenPath: next }) }}
           />
         </div>
+        <div className={css.row}>
+          <span className={css.rowText}>
+            <span className={css.title}>{t('settingsOpenToolsTitle')}</span>
+            <span className={css.desc}>{t('settingsOpenToolsDesc')}</span>
+          </span>
+          <Switch
+            label={t('settingsOpenToolsTitle')}
+            checked={prefs.agentOpenTools}
+            onChange={(next) => { applyPref({ agentOpenTools: next }) }}
+          />
+        </div>
         <div className={css.row}>
           <span className={css.rowText}>
             <span className={css.title}>{t('settingsTitleBarTitle')}</span>