Inconsistent terminology in skill-creator: assertions vs expectations

The skill seems to use the terms "assertions" and "expectations" interchangeably which is mostly fine... except I think it confuses the agent somewhat and results in an `evals.json` that uses `assertions` instead of `expectations` in the json.

Not sure what the consequences of this are but if some (non agentic) script tries to read this file and it doesn't match expectations then it might fail unnecessarily.