ability to disable `git` commands from opencode

[bczsalba]

Currently whenever you utter the word "undo" there is roughly a 50% chance that you get a context-aware undo. If you're unlucky everything you've not committed is down the drain before you have a chance to react. I have 0 use for opencode being able to run git commands, so I'd like to just disable them wholesale. I'm sure I could do some shell-level trickery to empty the git command in the opencode environment but that's too janky to rely on imo.

Couldn't find any related issues (besides snapshot stuff), nothing in config either.