Creates update-doomemacs.sh script that:
- Fetches latest commit SHA from doomemacs/doomemacs repo
- Automatically detects the default branch
- Updates both rev and sha256 in home/roles/emacs/default.nix
- Works from anywhere in the repo using git rev-parse
Also adds a flake app so it can be run with:
nix run .#update-doomemacs
