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/core/src/config/config_tests.rs

@@ -1261,6 +1261,7 @@ fn config_toml_deserializes_model_availability_nux() {
             vim_mode_default: false,
             question_esc_back: true,
             raw_output_mode: false,
+            fullscreen_transcript: false,
             alternate_screen: AltScreenMode::default(),
             status_line: None,
             status_line_use_colors: true,
@@ -4388,6 +4389,7 @@ fn tui_config_missing_notifications_field_defaults_to_enabled() {
             vim_mode_default: false,
             question_esc_back: true,
             raw_output_mode: false,
+            fullscreen_transcript: false,
             alternate_screen: AltScreenMode::Auto,
             status_line: None,
             status_line_use_colors: true,

codex-rs/core/src/config/mod.rs

@@ -770,6 +770,9 @@ pub struct Config {
     /// Start the TUI in raw scrollback mode for copy-friendly transcript output.
     pub tui_raw_output_mode: bool,
 
+    /// Own the fullscreen transcript when the alternate screen is enabled.
+    pub tui_fullscreen_transcript: bool,
+
     /// Start the TUI in the specified collaboration mode (plan/default).
 
     /// Controls whether the TUI uses the terminal's alternate screen buffer.
@@ -4406,6 +4409,10 @@ impl Config {
                 .as_ref()
                 .map(|t| t.raw_output_mode)
                 .unwrap_or(false),
+            tui_fullscreen_transcript: cfg
+                .tui
+                .as_ref()
+                .is_some_and(|tui| tui.fullscreen_transcript),
             tui_alternate_screen: cfg
                 .tui
                 .as_ref()