This should better be achieved by a gadget.

If you write a page in other namespace than User into the square brackets

There are valid reason to link a page in summary.

A warning should be shown when doing one of the following:

• If you write a page in other namespace than User into the square brackets
• […]

To be honest, I'm not sure whether this would work that well in practice, as people might deliberately want to link from an edit summary to a page that isn’t in the User: namespace.

I don't think people are using @ in links to mainspace pages

In T404450#11176611, @Hdicu wrote:

I don't think people are using @ in links to mainspace pages

Ah, I didn't mentally apply the @ you mentioned to the next thing you wrote in the task description, my bad.