[Feature request]: Specify an agents.yaml file for --agents mode.

### What do you need?

I'm looking for possibilities to specify agents behaviour in the fabric, When I tried to create agents for internet search task - always creates the test.yaml which always have empty tools. And as a result - creative halucinations in the response. I think it can be very convinient if you can call:
```fabric -a some_agents_file.yaml ```
or current behaviour if nothing provided