Actions are a resource in a Dominion that you can spend. It makes absolutely no sense to have negative Actions. You can only spend resources that you actually have.
And you're free to have your cards follow that rule if you wish. As of now, there is no ruling one way or another on whether Actions or Buys can go negative, so you can't really say whether it makes sense or not if there's no precedent. Actions are only a counter after all, and there's nothing inherent to them that demands non-negativity.
None of the 4 basic resources of the game can become negative. It is common-sensical, you can only spend stuff that you have.
Coins also cannot become negative, Debt is a totally different mechanism.
I'm not sure what your concern is, of course none of the 4 basic resources go negative in the base game, that's the entire point of fan mechanics. If it were already part of the game, then we wouldn't be discussing it here. Coins and the like cannot become negative because there are no cards with negative vanilla bonuses, that's the entire point of this contest. And regardless, the possibility of negative actions doesn't mean you can continue to use actions while negative to become even further negative. If you have 0 or less actions, you can't play any actions. Your statement "you can't spend what you don't have" isn't applicable here. None of these fan cards allow you to do that.
So your card implies that you can once go under zero with Actions but once you are in the negative realm you cannot play another card with -Actions.
Dude, that’s hyperunintuitive.
Spend X Actions is a fine mechanism. -X Actions with some weird „you can go once below zero but once you are you cannot play other -X Action cards“ rule is not.
Think about when you would go below zero. I never stated my card allows you to go below zero once. The rules around my card were "Do what the card says. If you do not have 1 or more actions afterwards, you cannot play another card", just like any other card. If you play a -1 Action card, that means you had 1 action, and were thus allowed to play a card, now having 0, do what the card says, and now have -1. You cannot play another action. It's exactly the same as playing any other terminal. You are not "allowed to go below once and then not again", you are simply allowed to play any action card if you have 1 action available, and if you are at 0 you can't play an action card period. It would be less intuitive if you couldn't play a -1 Action card while having 1 action. And anyway, the only time it matters whether or not you have 0 or -1 is when it comes to how many villagers you need to get back to 1 action. Once again, you are free to implement it however you wish.
Dude, you explicitly said that Steel Foundry means that you can end up with -1 Action and you just said it again. So according to you you can most definitely go below zero with the Action counter.
My point is that this is a total mess rule-wise (gee, the very fact that we have this discussion shows this). It makes far more sense to implement it Storyteller-style as „spend an Action“ which means that you need two Actions to play Steel Foundry.
Spending resources is cool, it is a basic mechanism familiar to anybody who plays Euros. But being able to spend stuff that you don’t have, man, just no. There is no precedent for this in Dominion, it will lead to quite some confusion and it also makes the card itself behave very weird (no idea about why you sting to it, all it achieves is make the card better suited for money).
by itself, imo. 
