I understand the concept of skills and agents. A skill is an ability to do something and an agent is something that does something. But what is an agent in a more "physical" sense? A skill is just a text markdown file written in English that can be understood, and stored in a folder in the Claude app. But what exacly is an agent? Is an agent is multiple skills plus timers. Would that be right? Where are they stored? What would an agent exactly be described as, in terms of the claude desktop app?