omm pin

Mark an installed model to be archived before its next `omm install --force`, so `omm rollback` can undo that reinstall afterwards. Nothing is copied yet - GGUFs are large, so the archive is only made right before the reinstall actually replaces the file, not at pin time. `omm install --force` still replaces a pinned model same as any other; pin only decides whether the version it replaces is kept.