I really hope this doesn't have the same security model as Chrome Extensions!
I can see the appeal of this feature and I am generally speaking an AI booster.
On the other hand...like...wat? This feature feels way too premature and risky to let loose on the public.
There are no third-party Skills, you can only create your own or use Google's readymade ones.
If you can create your own, what's stopping you from copy pasting someone else's?