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/engine/src/triage.ts

@@ -179,6 +179,7 @@ export class TriageProcessor {
   /** The interval (ms) of the currently active `setInterval` timer. */
   private activePollMs: number | null = null;
   private processing = new Set<string>();
+  private wasGlobalPaused = false;
 
   constructor(
     private store: TaskStore,
@@ -230,6 +231,16 @@ export class TriageProcessor {
       const settings = await this.store.getSettings();
       this.refreshPollInterval(settings.pollIntervalMs);
 
+      // Global pause: halt all triage activity
+      if (settings.globalPause) {
+        if (!this.wasGlobalPaused) {
+          triageLog.log("Global pause active — triage halted");
+          this.wasGlobalPaused = true;
+        }
+        return;
+      }
+      this.wasGlobalPaused = false;
+
       const tasks = await this.store.listTasks();
       const triageTasks = tasks.filter(
         (t) => t.column === "triage" && !this.processing.has(t.id) && !t.paused,