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>