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/engine/src/scheduler.ts

@@ -76,6 +76,7 @@ export class Scheduler {
   private scheduling = false;
   private wasWorktreeLimited = false;
   private wasGlobalPaused = false;
+  private wasEnginePaused = false;
   private pollInterval: ReturnType<typeof setInterval> | null = null;
   /** The interval (ms) of the currently active `setInterval` timer. */
   private activePollMs: number | null = null;
@@ -98,6 +99,18 @@ export class Scheduler {
         this.schedule();
       }
     });
+
+    /**
+     * Immediate soft-unpause resume: when `enginePaused` transitions from
+     * `true` to `false`, trigger a scheduling pass right away instead of
+     * waiting for the next poll interval. Same pattern as the globalPause
+     * unpause handler above.
+     */
+    this.store.on("settings:updated", ({ settings, previous }) => {
+      if (previous.enginePaused && !settings.enginePaused && this.running) {
+        this.schedule();
+      }
+    });
   }
 
   start(): void {
@@ -197,7 +210,7 @@ export class Scheduler {
       // Refresh the poll interval if the persisted setting has changed
       this.refreshPollInterval(settings.pollIntervalMs);
 
-      // Global pause: halt all scheduling activity
+      // Global pause (hard stop): halt all scheduling activity
       if (settings.globalPause) {
         if (!this.wasGlobalPaused) {
           schedulerLog.log("Global pause active — scheduling halted");
@@ -207,6 +220,16 @@ export class Scheduler {
       }
       this.wasGlobalPaused = false;
 
+      // Engine paused (soft pause): halt new work dispatch, but let agents finish
+      if (settings.enginePaused) {
+        if (!this.wasEnginePaused) {
+          schedulerLog.log("Engine paused — scheduling halted (in-flight agents continue)");
+          this.wasEnginePaused = true;
+        }
+        return;
+      }
+      this.wasEnginePaused = false;
+
       // Count only in-progress tasks toward the worktree limit.
       // In-review tasks with worktrees are idle (waiting to merge) and
       // should not block new tasks from starting.