[Contribution] ZSH Completion file

[obeone]

Hi,

I (no ok, not me, GPT-4) created a very performant ZSH completion file for sgpt command. You can find it here, fell free to add it to the project: https://gist.github.com/obeone/dc66f2ca40b8254edab61ac50cdec0f3

To give some context: While working on a custom GPT "ZSH Expert" that I am creating, I tested asking it for a completion for sgpt, and the result is surprisingly very good!