namespace Auth; class Auth_Login_Slauth extends \Auth_Login_Driver
Like you shouldn't extend the core within the core, you shouldn't extend a package within itself (unless of course it's your own package of some kind, than it's all up to you). Put the file in your app at fuel/app/classes/auth/login/slauth.php and remove the namespace decleration and it should work fine.
(and why it isn't found currently: classes within the Auth package need to be registered in the Auth's bootstrap to be found by the autoloader, but you should just extend in your app) It looks like you're new here. If you want to get involved, click one of these buttons!