I believe your comment above is correct, but subtle enough that both jacques_chester and whoever downvoted him may have mistakenly concluded that it's inconsistent with his comment below. The situation that I understand is that:

1. I sign a typical employment agreement where I promise to sign patent paperwork.

2. As part of my employment, I make a thing.

3. The company (my manager, the company's lawyers, etc.) wants to apply for a patent that claims my thing.

4. I agree that I made the thing, but I tell the company that the thing isn't patentable because of prior art.

Then what happens? Am I correct that (a) the company may proceed with a patent application; (b) the company is obligated to include me as an inventor, because I made the thing; (c) the company and I are obligated to disclose my alleged prior art with the application, assuming that my prior art really is in the general vicinity of my thing; and (d) I'm obligated to sign the declaration?

Nothing in any employment contract I've seen creates recourse for employees who disagree with the company about what's patentable, beyond waiting for the examiner to reject the claims. It would be weird if something did--the legal standard for prior art departs considerably from common sense, so I'm not sure why the non-lawyer employee's judgment would be relevant.