fix(models): make tier-change listener registration non-destructive

src/models/router.test.ts

@@ -463,4 +463,22 @@ describe('setClient and labels', () => {
     expect(fallback.chat).not.toHaveBeenCalled();
     expect(router.isTierStrict('default')).toBe(true);
   });
+
+  it('setOnTierChange does not replace existing listeners', () => {
+    const router = new ModelRouter({
+      default: { chat: vi.fn() } as unknown as ModelClient,
+      fast: { chat: vi.fn() } as unknown as ModelClient,
+      fallbackChain: [],
+    });
+
+    const first = vi.fn();
+    const second = vi.fn();
+    router.addOnTierChange(first);
+    router.setOnTierChange(second);
+
+    router.setTier('fast');
+
+    expect(first).toHaveBeenCalledWith('fast');
+    expect(second).toHaveBeenCalledWith('fast');
+  });
 });

src/models/router.ts

@@ -67,7 +67,7 @@ export class ModelRouter implements ModelClient {
   }
 
   setOnTierChange(callback: (tier: ModelTier) => void): void {
-    this.tierChangeListeners = [callback];
+    this.addOnTierChange(callback);
   }
 
   addOnTierChange(callback: (tier: ModelTier) => void): void {