Move fullscreen transcript control to TUI configuration (#46849)

## What changed

Use `tui.fullscreen_transcript` to opt into the fullscreen transcript, including scrolling, selection, and search. It defaults to `false`; `--no-alt-screen` and `tui.alternate_screen = "never"` still take precedence.

The new preference controls both the first frame and application startup. Boolean CLI overrides for it are allowed during daemon startup. The old `features.transcript_v2` flag is deprecated and ignored, with a notice directing users to the new setting; it does not migrate into or override the preference.

## Testing

Add coverage for default terminal scrollback, fullscreen persistence through startup, alternate-screen restrictions, daemon override validation, and independence from the deprecated flag. Update existing history and activity tests to use the new preference.

GitOrigin-RevId: 4117bf1ee548fa8d101a594f9506673d87c74c15

codex-rs/thread-manager-sample/src/main.rs

@@ -232,6 +232,7 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         tui_show_server_version_notice: true,
         tui_auto_recap: true,
         model_availability_nux: ModelAvailabilityNuxConfig::default(),
+        tui_fullscreen_transcript: false,
         tui_alternate_screen: AltScreenMode::Auto,
         tui_status_line: None,
         tui_status_line_use_colors: true,