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

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.

Start by reproducing copilot --no-alt-screen and inspect /settings for the current alt-screen controls. Trace the CLI option and settings entry points to determine whether an equivalent exists or whether the flag must return. Done means inline or streaming rendering can be disabled through a supported, documented interface without the unknown-option failure.