I work on Bun (though I did not work on this feature much outside of reviewing the PR), happy to answer any questions

Why pick this over https://github.com/google/zx?

Yeah, or over https://github.com/sindresorhus/execa?

And given the existence of those npm packages, is there any aspect of Bun Shell that required it to be built into the Bun runtime instead of published to npm?