Add ⌘K Clear to Start for the integrated terminal

[Borisserz]

Description

Implements ⌘K in the integrated terminal so it clears the screen and the scrollback, the same way Terminal.app and VS Code do it.

I hooked this up on the terminal view itself: when it has focus and the option is on, ⌘K runs a clear that only touches the emulator buffer (nothing goes to the shell). There’s a new toggle in Terminal Settings — Clear to Start with ⌘K — and it’s on by default, which matches what #891 asked for.

Related Issues

• closes ✨ Add "Clear To Start" (⌘K) shortcut to integrated terminal #891

Checklist

• I read and understood the contributing guide as well as the code of conduct
• The issues this PR addresses are related to each other
• My changes generate no new warnings
• My code builds and runs on my machine
• My changes are all related to the related issue above
• I documented my code

Screenshots

No layout changes besides the new settings toggle.

Test plan

• Unit tests cover the setting defaults/decoding and that clearToStart() wipes viewport + scrollback
• Focus the terminal, print some output, hit ⌘K — screen and scrollback should be empty
• Turn off Clear to Start with ⌘K — ⌘K shouldn’t clear anymore
• Confirm ⌘K doesn’t do anything weird when the editor (not the terminal) is focused