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.

README_EN.md

@@ -43,6 +43,7 @@
 - **🗂️ File Workbench**: file explorer (lazy-loading tree; symlinks show their target kind — directory links expand, dangling links flagged) + CodeMirror editor; inline preview for images / Markdown (incl. Mermaid diagrams, strict-mode safe rendering + click-to-zoom) / HTML / PDF
 - **🌐 Embedded Browser**: multiple web tabs with back / forward / refresh; content runs in a sandboxed iframe; external links are routed by protocol by default — HTTP opens in the sidebar, HTTPS goes to the system browser (both adjustable in settings)
 - **💻 Real Terminal**: xterm.js + node-pty real shell, reconnect with transcript replay; optionally injects `terminal_*` tools for the model
+- **📂 Model-driven sidebar opens (opt-in)**: with the global setting on, the `sidebar_open` tool lets the model actively open files / folders (tree rooted there) / HTTP(S) pages in the sidebar
 - **🌿 Git Panel**: real diff + VSCode-style diff tabs, history, right-click to stage / commit / revert
 - **🧩 Background Tasks**: agent topology + background tasks (exit codes / live output / force-kill)
 - **💬 Side Chat (beta)**: Codex-style side threads — the child inherits the parent's FULL context (completed turns + the pending question + the in-progress turn's assistant output and tool activity, honestly frozen as "interrupted") and runs independently without entering the main conversation; threads support continuous follow-ups (auto-resumed after a DSH restart) and one-click "Save as new session" promotion to a top-level session