feat: implement double Ctrl+C to exit functionality with warning toast

[AmineGuitouni]

Problem: It's easy to accidentally exit opencode by pressing Ctrl+C once.

Proposed Solution: Require pressing Ctrl+C twice within 2 seconds to exit. On first press, show a toast warning "Press again to exit".

Implementation: I have a working implementation ready if this is approved.

[shuv1337]

@rekram1-node - this one has been really useful as well. i struggle with accidentally closing opencode way too much, especially since control-c also clears the prompt. this gives a nice buffer while retaining existing functionality