I think skill is the perfect name for this. You provide the LLM with a new skill by telling it how to do a thing and providing supporting scripts to help it do that thing.

Yup! I fully agree. It also taps into the ability of LLMs to write code given good prompts. All you need is for the LLM to recognize that it needs something, fetch it into the context, and write exactly the code that is needed in the current combination of skill + previous context.

You've described instructions. It already had a name.