--no-alt-screen was silently removed with no replacement — alt-screen is now unavoidable and broken

[bounis]

Alt-screen/fullscreen mode is a genuine regression for a lot of us, and it's
been reported since March (#1799, #2334) with no resolution.

What makes this worse: the --no-alt-screen flag that used to let people
opt out has now been removed entirely, with no deprecation notice and no
replacement setting. Running it now just fails:

$ copilot --no-alt-screen
error: unknown option '--no-alt-screen'

I checked /settings for an equivalent key and found nothing that restores
inline/streaming rendering. So as of the current release, there is no
supported way to turn off fullscreen mode at all.

This isn't a nice-to-have. Alt-screen breaks:

• native terminal scrollback (can't scroll up past the current screen)
• terminal search (Cmd+F / Ctrl+F can't see history)
• copy/paste in some terminal emulators
• usability in tmux/screen and browser-based terminals

Please either:

1. Restore --no-alt-screen (or an equivalent /settings key), or
2. Clearly document the replacement if one exists and I'm missing it.

Removing an escape hatch that people were actively relying on, without
warning, is a bad pattern. At minimum this needs a documented workaround.