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/config/src/types.rs

@@ -793,6 +793,11 @@ pub struct Tui {
     #[serde(default)]
     pub raw_output_mode: bool,
 
+    /// Own the fullscreen transcript, including scrolling, selection, and search.
+    /// Defaults to `false`; alternate-screen restrictions take precedence.
+    #[serde(default)]
+    pub fullscreen_transcript: bool,
+
     /// Controls whether the TUI uses the terminal's alternate screen buffer.
     ///
     /// - `auto` (default): Use alternate screen.