feat(KB-145): add global pause button to halt all AI engine activity

- Add globalPause setting to core Settings type with default false
- Guard scheduler, triage, and auto-merge queue to skip work when globalPause is active
- Add pause/play toggle button in dashboard Header with optimistic UI and rollback on failure
- Add tests for scheduler, triage, Header, and App global pause behavior
- Include minor changeset for the new feature

packages/dashboard/app/components/Header.tsx

@@ -1,10 +1,12 @@
-import { Settings } from "lucide-react";
+import { Settings, Pause, Play } from "lucide-react";
 
 interface HeaderProps {
   onOpenSettings?: () => void;
+  globalPaused?: boolean;
+  onToggleGlobalPause?: () => void;
 }
 
-export function Header({ onOpenSettings }: HeaderProps) {
+export function Header({ onOpenSettings, globalPaused, onToggleGlobalPause }: HeaderProps) {
   return (
     <header className="header">
       <div className="header-left">
@@ -13,6 +15,13 @@ export function Header({ onOpenSettings }: HeaderProps) {
         <span className="logo-sub">board</span>
       </div>
       <div className="header-actions">
+        <button
+          className={`btn-icon${globalPaused ? " btn-icon--paused" : ""}`}
+          onClick={onToggleGlobalPause}
+          title={globalPaused ? "Resume AI engine" : "Pause AI engine"}
+        >
+          {globalPaused ? <Play size={16} /> : <Pause size={16} />}
+        </button>
         <button className="btn-icon" onClick={onOpenSettings} title="Settings">
           <Settings size={16} />
         </button>

packages/dashboard/app/components/__tests__/App.test.tsx

@@ -44,7 +44,7 @@ vi.mock("../../hooks/useTasks", () => ({
   }),
 }));
 
-import { fetchAuthStatus, fetchSettings } from "../../api";
+import { fetchAuthStatus, fetchSettings, updateSettings } from "../../api";
 
 beforeEach(() => {
   vi.clearAllMocks();
@@ -132,3 +132,79 @@ describe("App auto-open Settings on unauthenticated", () => {
     expect(screen.queryByText("Anthropic")).toBeNull();
   });
 });
+
+describe("App global pause", () => {
+  it("initializes global pause state from fetchSettings", async () => {
+    (fetchSettings as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+      ...defaultSettings,
+      globalPause: true,
+    });
+
+    render(<App />);
+
+    // When globally paused, the button should show "Resume AI engine"
+    await waitFor(() => {
+      expect(screen.getByTitle("Resume AI engine")).toBeTruthy();
+    });
+  });
+
+  it("shows Pause button when globalPause is false", async () => {
+    (fetchSettings as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+      ...defaultSettings,
+      globalPause: false,
+    });
+
+    render(<App />);
+
+    await waitFor(() => {
+      expect(screen.getByTitle("Pause AI engine")).toBeTruthy();
+    });
+  });
+
+  it("toggles global pause state and calls updateSettings", async () => {
+    (fetchSettings as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+      ...defaultSettings,
+      globalPause: false,
+    });
+
+    render(<App />);
+
+    // Wait for initial render
+    await waitFor(() => {
+      expect(screen.getByTitle("Pause AI engine")).toBeTruthy();
+    });
+
+    // Click the pause button
+    fireEvent.click(screen.getByTitle("Pause AI engine"));
+
+    // Should optimistically switch to "Resume" state
+    await waitFor(() => {
+      expect(screen.getByTitle("Resume AI engine")).toBeTruthy();
+    });
+
+    // Should call updateSettings with globalPause: true
+    expect(updateSettings).toHaveBeenCalledWith({ globalPause: true });
+  });
+
+  it("reverts global pause state on updateSettings failure", async () => {
+    (fetchSettings as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+      ...defaultSettings,
+      globalPause: false,
+    });
+    (updateSettings as ReturnType<typeof vi.fn>).mockRejectedValueOnce(new Error("Network error"));
+
+    render(<App />);
+
+    await waitFor(() => {
+      expect(screen.getByTitle("Pause AI engine")).toBeTruthy();
+    });
+
+    // Click the pause button — will fail
+    fireEvent.click(screen.getByTitle("Pause AI engine"));
+
+    // Should revert back to "Pause" state after failure
+    await waitFor(() => {
+      expect(screen.getByTitle("Pause AI engine")).toBeTruthy();
+    });
+  });
+});

packages/dashboard/app/components/__tests__/Header.test.tsx

@@ -1,5 +1,5 @@
 import { describe, it, expect, vi } from "vitest";
-import { render, screen } from "@testing-library/react";
+import { render, screen, fireEvent } from "@testing-library/react";
 import { Header } from "../Header";
 
 describe("Header", () => {
@@ -25,4 +25,36 @@ describe("Header", () => {
     const btn = screen.getByTitle("Settings");
     expect(btn).toBeDefined();
   });
+
+  it("renders pause button with 'Pause AI engine' title when not paused", () => {
+    render(<Header globalPaused={false} />);
+    const btn = screen.getByTitle("Pause AI engine");
+    expect(btn).toBeDefined();
+  });
+
+  it("renders play button with 'Resume AI engine' title when paused", () => {
+    render(<Header globalPaused={true} />);
+    const btn = screen.getByTitle("Resume AI engine");
+    expect(btn).toBeDefined();
+  });
+
+  it("calls onToggleGlobalPause when pause button is clicked", () => {
+    const onToggle = vi.fn();
+    render(<Header globalPaused={false} onToggleGlobalPause={onToggle} />);
+    const btn = screen.getByTitle("Pause AI engine");
+    fireEvent.click(btn);
+    expect(onToggle).toHaveBeenCalledOnce();
+  });
+
+  it("applies btn-icon--paused class when paused", () => {
+    render(<Header globalPaused={true} />);
+    const btn = screen.getByTitle("Resume AI engine");
+    expect(btn.className).toContain("btn-icon--paused");
+  });
+
+  it("does not apply btn-icon--paused class when not paused", () => {
+    render(<Header globalPaused={false} />);
+    const btn = screen.getByTitle("Pause AI engine");
+    expect(btn.className).not.toContain("btn-icon--paused");
+  });
 });