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/__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");
+  });
 });