Trust

[Dom R]

I will go against the grain, and say, that if she brings it up again, you should reply, that you care about your kids, and that decisions about their future welfare should be made jointly.

(that is to say, gently imply that if she ever divorces you, no way you are letting her take sole custody. Without actually bringing up fighting, or divorce.)

I take it from the overall tone of your letter, that you definately would fight her over it.
So, rather than say "yes i would fight you over it", just re-emphasise that you are BOTH their parents, rather than she being the sole parent.

Giving your spouse the impression that you wouldnt fight for joint custody if she filed, is a big mistake.
If she does file, then she will be all the angrier when you fight her, "unexpectedly".
And also, it may give her second thoughts about filing in the first place