Whether you are or are not entitled to overtime when you work over 40 hours a week is determined by Federal law, not by agreement. I do not see that your agreement guarantees you 40 hours a week; only that 40 hours is their standard. (As opposed to my employer, whose standard is 35 hours, or my last employer, whose standard was 37.5.)
Therefore, no, I do not think that they have either breached the agreement or nullified the sign on bonus agreement.)
Two things I am tired of typing: 1.) A wrongful termination does not mean that you were fired for something you didn't do; it means that you were fired for a reason prohibited by law. 2.) The above answer, whatever it is, assumes that no legally binding contract or CBA expressly says otherwise. If it does, the terms of the contract apply.