What atones for a widow not staying in her husband's house for four months and ten days, especially if her leaving is to provide food for her livestock?

There is no harm for the iddah woman to leave her house to bring food for her sheep if she cannot find anyone else to do so, and there is no expiation required of her. However, if she leaves without a need, the expiation for that is repentance and seeking forgiveness if she was aware of the prohibition.