Call custom agent failed from another custom agent in copilot CLI, but it works for coding agent

Hi,

I have a custom agent which is a plan agent, which will call other custom agent to execution specific task, when I call other agent from the plan agent, I will got the error like

```
Failure Reason: The custom agent invocation mechanism required to dispatch to specialized agents (xx-agent1, xx-agent2) is not available in the current environment
```

But it works in coding agent