Mental entities can be modeled as Agents — with a utility function.

Thermodynamics can't. It's an entropic process, all it does is uniformly increase entropy in the universe. It does not do work in order to shape reality into a set of desirable states.

In fact all basic laws of (continuous, Hamiltonian (if I got that one right)) physics are entropy increasing and are only poorly modeled as agents.

Humans are much better modeled as agents. As are fictional deities and spirits.

An ontologically fundamental mental entity is 1) inherently indivisible (like an electron or a photon) and 2) an agent.