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/locales-nl.ts

@@ -183,6 +183,8 @@ export const nl: Record<string, string> = {
   settingsWidthSuffix: '%',
   settingsOpenPathTitle: 'Chatbestanden in de zijbalk openen',
   settingsOpenPathDesc: 'Bestandslinks in de chat (toolrijen, geproduceerde bestanden, vermeldingen) openen in de zijbalk-editor in plaats van de standaard systeemapp',
+  settingsOpenToolsTitle: 'Zijbalk-openen-tool voor het model injecteren',
+  settingsOpenToolsDesc: 'Indien ingeschakeld kan het model bestanden, mappen en HTTP(S)-pagina\'s in de zijbalk openen via de sidebar_open-tool (standaard uit)',
   settingsTitleBarTitle: 'Positiecompatibiliteitsmodus',
   settingsTitleBarDesc: 'Kies het titelbalk-compatibiliteitsschema: auto-detect (standaard, conservatief) / DSH officiële web / bekende desktop-shells / aangepast (verschuivingsafstand + aangepaste CSS)',
   settingsTitleBarStripTitle: 'Verschuivingsafstand',