feat(KB-157): add soft-pause (enginePaused) alongside hard-stop (globalPause)

- Add enginePaused field to Settings type with soft-pause semantics (drain queue, don't kill agents)
- Gate scheduler, triage, auto-merge, and periodic merge retry on enginePaused
- Add Pause and Stop buttons to dashboard Header replacing single toggle
- Wire enginePaused state through App with optimistic toggle and unpause resume logic
- Add tests for scheduler, triage, dashboard CLI, Header, and App covering pause behavior

packages/dashboard/app/components/Header.tsx

@@ -1,12 +1,20 @@
-import { Settings, Pause, Play } from "lucide-react";
+import { Settings, Pause, Play, Square } from "lucide-react";
 
 interface HeaderProps {
   onOpenSettings?: () => void;
   globalPaused?: boolean;
+  enginePaused?: boolean;
   onToggleGlobalPause?: () => void;
+  onToggleEnginePause?: () => void;
 }
 
-export function Header({ onOpenSettings, globalPaused, onToggleGlobalPause }: HeaderProps) {
+export function Header({
+  onOpenSettings,
+  globalPaused,
+  enginePaused,
+  onToggleGlobalPause,
+  onToggleEnginePause,
+}: HeaderProps) {
   return (
     <header className="header">
       <div className="header-left">
@@ -15,12 +23,22 @@ export function Header({ onOpenSettings, globalPaused, onToggleGlobalPause }: He
         <span className="logo-sub">board</span>
       </div>
       <div className="header-actions">
+        {/* Pause button (soft pause): stops new work, lets agents finish */}
         <button
-          className={`btn-icon${globalPaused ? " btn-icon--paused" : ""}`}
-          onClick={onToggleGlobalPause}
-          title={globalPaused ? "Resume AI engine" : "Pause AI engine"}
+          className={`btn-icon${enginePaused ? " btn-icon--paused" : ""}`}
+          onClick={onToggleEnginePause}
+          title={enginePaused ? "Resume AI engine" : "Pause AI engine"}
+          disabled={!!globalPaused}
         >
-          {globalPaused ? <Play size={16} /> : <Pause size={16} />}
+          {enginePaused ? <Play size={16} /> : <Pause size={16} />}
+        </button>
+        {/* Stop button (hard stop): kills all agents immediately */}
+        <button
+          className={`btn-icon${globalPaused ? " btn-icon--stopped" : ""}`}
+          onClick={onToggleGlobalPause}
+          title={globalPaused ? "Start AI engine" : "Stop AI engine"}
+        >
+          {globalPaused ? <Play size={16} /> : <Square size={16} />}
         </button>
         <button className="btn-icon" onClick={onOpenSettings} title="Settings">
           <Settings size={16} />