Here's the thing. As you're indicating, UI and UX are not the same thing, but they're both subjective.

You can ask the model to implement common patterns for both UI and UX. You can ask the model to correct obvious UI errors.

But you can't really ask the model to have UI taste unless you supply that taste, and you can't ask a model to "do UX" in a vacuum. UX work requires user testing. So any UX workflow that is going to be handled by agents must also include observational data of humans using the software.

There is no set of instructions that can do "UI/UX" without also incorporating human feedback.