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.

tests/plugin-shape.spec.ts

@@ -65,6 +65,8 @@ describe('dsh-better-sidebar plugin export shape', () => {
     // The terminal tools default OFF (the feature is dormant until the user
     // enables it in the side card settings).
     expect(resolved.agentTerminalTools).toBe(false)
+    // The sidebar-open tool defaults OFF too (same dormant-until-enabled rule).
+    expect(resolved.agentOpenTools).toBe(false)
     // The terminal font customizations default to the theme (empty family)
     // and 13px.
     expect(resolved.terminalFontFamily).toBe('')
@@ -92,6 +94,6 @@ describe('dsh-better-sidebar plugin export shape', () => {
     const overridden = (PrefsSchema as unknown as {
       (input: Record<string, unknown> | undefined): Record<string, unknown>
     })({ openByDefault: false, defaultWidthPercent: 45 })
-    expect(overridden).toEqual({ openByDefault: false, defaultWidthPercent: 45, autoOpenSubagent: true, autoOpenJobs: true, agentTerminalTools: false, bottomPanelAutoTerminal: true, terminalFontFamily: '', terminalFontSize: 13, interceptOpenPath: true, editorExplorer: false, terminalShell: '', terminalShellArgs: '', titleBarCompat: false, titleBarStripPx: 40, htmlViewerNoSandbox: false, htmlViewerDefaultUnsafe: false, browserNoSandbox: false, browserInterceptLinks: true, browserInterceptHttp: true, browserInterceptHttps: false, tabsEnabled: {}, viewersEnabled: {}, pluginSettings: {} })
+    expect(overridden).toEqual({ openByDefault: false, defaultWidthPercent: 45, autoOpenSubagent: true, autoOpenJobs: true, agentTerminalTools: false, agentOpenTools: false, bottomPanelAutoTerminal: true, terminalFontFamily: '', terminalFontSize: 13, interceptOpenPath: true, editorExplorer: false, terminalShell: '', terminalShellArgs: '', titleBarCompat: false, titleBarStripPx: 40, htmlViewerNoSandbox: false, htmlViewerDefaultUnsafe: false, browserNoSandbox: false, browserInterceptLinks: true, browserInterceptHttp: true, browserInterceptHttps: false, tabsEnabled: {}, viewersEnabled: {}, pluginSettings: {} })
   })
 })