What should the wife do if her husband brings back his first wife after a prior agreement not to bring her back, and is she entitled to seek a divorce due to his breaking of the covenant?

If you stipulated at the time of the marriage contract that your affair would be in your hands (meaning you have the right to divorce) if your husband does not adhere to the condition of not taking back his first wife, then you have the option (to separate) if he does not fulfill the condition. However, it is not advisable to rush into divorce. If you did not stipulate that, then you do not have the option. Seeking divorce is not permissible without necessity, for "any woman who asks her husband for divorce without harm (or necessity), the fragrance of Paradise will be forbidden to her." And a husband forgiving his wife is a commendable act with no fault in it, so do not preoccupy yourself with this matter.