feature: Remove exgentic

### Feature Description

https://github.com/rossoctl/examples/tree/main/a2a/exgentic_agent does not have usable instructions.



### Proposed Solution

Remove code and leave a README pointing to https://github.com/rossoctl/workload-harness/

### Want to contribute?

- [ ] I would like to work on this issue.

### Additional Context

_No response_