fix(core): fire statusChange in MonitorRegistry.reset()

The newly-added `setStatusChangeCallback` subscriber misses `reset()`,
so a `/clear` or session reset leaves stale monitor rows visible in the
combined Background tasks dialog until an unrelated register/settle
event happens. Both BackgroundShellRegistry and BackgroundTaskRegistry
already fire statusChange on their reset paths — Monitor was the
outlier.

Fix: fire `statusChange()` (no arg) after `monitors.clear()`, with an
early return when the registry is already empty so we don't notify on
a no-op reset. Two new tests cover both branches.

packages/core/src/services/monitorRegistry.test.ts

@@ -664,5 +664,22 @@ describe('MonitorRegistry', () => {
       ).not.toThrow();
       expect(registry.get('a')).toBeDefined();
     });
+
+    it('fires once on reset() so dialog snapshots clear stale rows', () => {
+      registry.register(createEntry({ monitorId: 'a' }));
+      registry.register(createEntry({ monitorId: 'b' }));
+      const cb = vi.fn();
+      registry.setStatusChangeCallback(cb);
+      registry.reset();
+      expect(cb).toHaveBeenCalledTimes(1);
+      expect(registry.getAll()).toEqual([]);
+    });
+
+    it('reset() on an empty registry does not fire statusChange', () => {
+      const cb = vi.fn();
+      registry.setStatusChangeCallback(cb);
+      registry.reset();
+      expect(cb).not.toHaveBeenCalled();
+    });
   });
 });

packages/core/src/services/monitorRegistry.ts

@@ -250,6 +250,7 @@ export class MonitorRegistry {
   }
 
   reset(): void {
+    if (this.monitors.size === 0) return;
     for (const entry of this.monitors.values()) {
       this.clearIdleTimer(entry);
       if (entry.status === 'running') {
@@ -257,6 +258,12 @@ export class MonitorRegistry {
       }
     }
     this.monitors.clear();
+    // Notify subscribers that the registry's contents changed wholesale
+    // — without this, the dialog snapshot in `useBackgroundTaskView`
+    // would keep rendering the now-cleared rows until an unrelated
+    // register/settle event happens. Mirrors BackgroundShellRegistry /
+    // BackgroundTaskRegistry's reset paths.
+    this.fireStatusChange();
   }
 
   // --- Internal helpers ---