I think this is neat! Love the cute lil' characters, and I believe that simplified build systems are still fertile ground. Filtering and deriving from collections will be essential for larger projects.

> $_IN: [...] For collections, this is a space separated list of filenames > [...] > I haven’t written any software for this

Bit early for this sort of nit-picking, but that will need to be either a.) an escaped and quoted list or b.) an argv array that doesn't undergo word-splitting.

Bonus consideration: What if the user's shell isn't sh? Perhaps it's out of scope (ie. this shouldn't support every use case of Make), but for what it's worth, they support all sorts of recipe interpreters.