What is an agent harness?

The other day, I used the phrase "agent harness," and my colleague asked me if I had an equestrian show on in the background. I didn't (that time). But it made me realize that not everyone is as deep in the weeds of AI tooling as I am, and that a plain-English explanation was in order. Here, I'll tell you what agent harnesses are and how they're different from models and agents. I'll also make the case for interoperability—why you don't want your app connections, context, and governance trapped