People think a Skill is some complicated build. It isn't.
Open one up and it's a markdown file. It's literally called SKILL.md. The only thing on top is four lines: a name, and a description of what it's for and when to use it.
That description is the trigger. It's the part Claude reads to decide whether this is the moment to open it.
Everything below it is just your instructions, written like you'd explain it to a person.
✦ A plain .md file has the instructions
✦ A Skill has the instructions plus the trigger
That's the entire upgrade. Four lines.