This adds a new backupCommand option to allow users to specify a
custom command to run on existing files during activation, as an
alternative to the existing backupFileExtension mechanism.
Adds backupCommand option to NixOS and nix-darwin modules.
Exports HOME_MANAGER_BACKUP_COMMAND environment variable when set.
Updates file activation logic to use the custom backup command if
provided, falling back to the existing backup extension logic.
Updates collision checking and user-facing instructions to
mention the new option.
This enables advanced backup workflows, such as moving files to
trash or archiving with custom tools, before managing them with
Home Manager.
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