The nomenclature that makes sense for me is that the agent is the combination of the harness and the model. The model provides text-completion, the harness provides the loop around it, and the agent is the full structure of both.

However, nomenclature evolves over time. I recall (perhaps falsely) that The Cloud was specifically a term for elastic on-demand provider-managed compute/storage/network. Over time, it came to mean many other things. e.g. Salesforce Data Cloud.

I imagine if you step away from this for a year and come back, an agent will be something entirely different, perhaps a robotic horse, and a harness will be your saddle on the horse. Who knows?