How would the AI know about the calendar app unless you make the text file and attach it to the session?

Self-describing APIs require probing through calls, they don't tell you what you need to know before you interact with them.

MCP servers are very simple to implement, and the developers of the app/service maintain the server so you don't have to create or update skills with incomplete understanding of the system.

Your skill file is going to drift from the actual API as the app updates. You're going to have to manage it, instead of the developers of the app. I don't understand what you're even talking about.