libghostty-vt: preserve shell prompts on resize by default

[noib3]

This PR makes libghostty-vt preserve shell prompts across resize unless the shell explicitly opts into prompt clearing/redraw with redraw=1.

[mitchellh]

Darn, I can't remember the history here of why this is even defaulted to true. It seems like it might be safe to default this to false in all of Ghostty until we see any semantic prompt marker. I want to think about that for a bit instead of the targeted fix here in libghostty-vt.

[jparise]

Darn, I can't remember the history here of why this is even defaulted to true. It seems like it might be safe to default this to false in all of Ghostty until we see any semantic prompt marker. I want to think about that for a bit instead of the targeted fix here in libghostty-vt.

Given that ;redraw is a Kitty invention (not in OSC 133 itself) to opt'ing in to shell redraws, I agree it makes sense to default this to false in the entire runtime.