Set home-manager.backupFileExtension = "backup" on the NixOS and Darwin base modules. Existing machines carry hand-written ~/.zshrc and ~/.zshenv that home-manager would refuse to overwrite, aborting activation. With a backup extension, the originals are saved as .backup and activation proceeds. The oh-my-zsh setup is already declared in lyrathorpe/home/shell.nix, so the generated files supersede the old ones.
Emma Thorpe