chore: add version recipe to justfile

Run 'just version' to see current GITMUX_VERSION.

Co-authored-by: stavxyz <hi@stav.xyz>

Merge pull request #15 from stavxyz/update-github-cli

Update GitHub CLI to 1.13.1

submit_pr is a bool!

fix _cmd_exists invocation

makefile fix

add cleanup function in makefile for lingering test repos