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/scheduler.ts

@@ -75,6 +75,7 @@ export class Scheduler {
   private running = false;
   private scheduling = false;
   private wasWorktreeLimited = false;
+  private wasGlobalPaused = false;
   private pollInterval: ReturnType<typeof setInterval> | null = null;
   /** The interval (ms) of the currently active `setInterval` timer. */
   private activePollMs: number | null = null;
@@ -181,6 +182,16 @@ export class Scheduler {
       // Refresh the poll interval if the persisted setting has changed
       this.refreshPollInterval(settings.pollIntervalMs);
 
+      // Global pause: halt all scheduling activity
+      if (settings.globalPause) {
+        if (!this.wasGlobalPaused) {
+          schedulerLog.log("Global pause active — scheduling halted");
+          this.wasGlobalPaused = true;
+        }
+        return;
+      }
+      this.wasGlobalPaused = 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.