Sigil compiles a plain-markdown skill into a program the model runs inside. It's not a runtime that reads skills, it's a compiler: SKILL.md in, standalone agent harness out.
It's written in Jac (https://www.jaseci.org), that's where the graph runtime, the typed "by llm" slots and the concurrency come from. Docs: https://sigilagent.com. Happy to go deep on the IR design.