Renoise is the tracker I've used since 2006. rnz puts it behind a command line, so an agent can analyze patterns, write notes and chords, load samples, add DSP, automate parameters, and build a full arrangement without touching the mouse.

The CLI speaks JSON to a Lua tool running inside Renoise. Commands are colocated: each one is a folder with the CLI half and the Renoise half side by side.