feat(companion): add idle observability alias

src/companion/runtimeClient.test.ts

@@ -632,15 +632,18 @@ describe('CompanionRuntimeClient', () => {
     await client.connect();
     expect(client.pendingRequestCount).toBe(0);
     expect(client.hasPendingWork).toBe(false);
+    expect(client.idle).toBe(true);
 
     const pending = client.call('system.capabilities');
     expect(client.pendingRequestCount).toBe(1);
     expect(client.hasPendingWork).toBe(true);
+    expect(client.idle).toBe(false);
 
     client.disconnect();
     await expect(pending).rejects.toThrow('Disconnected');
     expect(client.pendingRequestCount).toBe(0);
     expect(client.hasPendingWork).toBe(false);
+    expect(client.idle).toBe(true);
   });
 
   it('returns pending work snapshot', async () => {