Principle Model the Domain skill: what it does and how to install it

What it does. Guides code changes towards structures such as state machines, typed models, registries, reducers and normalized collections that encode domain rules directly. It looks for repeated branches, shape assumptions and lifecycle risks, while avoiding abstractions that add indirection without simplifying the code.

When to use it. For stateful logic, growing conditional chains or repeated assumptions about data shape across files. It favours clear, local code when the existing design is unlikely to grow.