Autocompletion for a just script?

I've long had a [`setup`](https://github.com/kbd/setup/blob/main/HOME/bin/setup) program that makes symlinks to dotfiles, installs packages, etc. At some point I realized I could replace my code with `just` (thanks for that), so I rewrote it in terms of a `justfile`.

Now, I want autocomplete for my `setup` script. I tried making it a [just script](https://github.com/casey/just?tab=readme-ov-file#just-scripts) hoping that just+zsh would work some magic, but no such luck. I tried `eval $(setup --completions zsh)` on that just script, but nope. Is there a way of getting zsh to complete a just script? It works how I expect when doing `just [tab]` when there's a justfile in the current directory.

Start with the linked just-script documentation and the existing zsh completion command shown in the issue. Investigate how completion works for just in a directory with a justfile versus for the standalone setup script. Done means setup [tab] provides useful recipe completions, or the supported limitation is documented.

Written by the indexing model from the issue text.