fix: add tui.alternate_screen config and --no-alt-screen CLI flag for Zellij scrollback (#8555)

Fixes #2558

Codex uses alternate screen mode (CSI 1049) which, per xterm spec,
doesn't support scrollback. Zellij follows this strictly, so users can't
scroll back through output.

**Changes:**
- Add `tui.alternate_screen` config: `auto` (default), `always`, `never`
- Add `--no-alt-screen` CLI flag
- Auto-detect Zellij and skip alt screen (uses existing `ZELLIJ` env var
detection)

**Usage:**
```bash
# CLI flag
codex --no-alt-screen

# Or in config.toml
[tui]
alternate_screen = "never"
```

With default `auto` mode, Zellij users get working scrollback without
any config changes.

---------

Co-authored-by: Josh McKinney <joshka@openai.com>

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

@@ -32,6 +32,7 @@ use crate::protocol::AskForApproval;
 use crate::protocol::SandboxPolicy;
 use codex_app_server_protocol::Tools;
 use codex_app_server_protocol::UserSavedConfig;
+use codex_protocol::config_types::AltScreenMode;
 use codex_protocol::config_types::ForcedLoginMethod;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::config_types::SandboxMode;
@@ -236,6 +237,14 @@ pub struct Config {
     /// consistently to both mouse wheels and trackpads.
     pub tui_scroll_invert: bool,
 
+    /// Controls whether the TUI uses the terminal's alternate screen buffer.
+    ///
+    /// This is the same `tui.alternate_screen` value from `config.toml` (see [`Tui`]).
+    /// - `auto` (default): Disable alternate screen in Zellij, enable elsewhere.
+    /// - `always`: Always use alternate screen (original behavior).
+    /// - `never`: Never use alternate screen (inline mode, preserves scrollback).
+    pub tui_alternate_screen: AltScreenMode,
+
     /// The directory that should be treated as the current working directory
     /// for the session. All relative paths inside the business-logic layer are
     /// resolved against this path.
@@ -1443,6 +1452,11 @@ impl Config {
                 .as_ref()
                 .and_then(|t| t.scroll_wheel_like_max_duration_ms),
             tui_scroll_invert: cfg.tui.as_ref().map(|t| t.scroll_invert).unwrap_or(false),
+            tui_alternate_screen: cfg
+                .tui
+                .as_ref()
+                .map(|t| t.alternate_screen)
+                .unwrap_or_default(),
             otel: {
                 let t: OtelConfigToml = cfg.otel.unwrap_or_default();
                 let log_user_prompt = t.log_user_prompt.unwrap_or(false);
@@ -1641,6 +1655,7 @@ persistence = "none"
                 scroll_wheel_tick_detect_max_ms: None,
                 scroll_wheel_like_max_duration_ms: None,
                 scroll_invert: false,
+                alternate_screen: AltScreenMode::Auto,
             }
         );
     }
@@ -3276,6 +3291,7 @@ model_verbosity = "high"
                 tui_scroll_wheel_tick_detect_max_ms: None,
                 tui_scroll_wheel_like_max_duration_ms: None,
                 tui_scroll_invert: false,
+                tui_alternate_screen: AltScreenMode::Auto,
                 otel: OtelConfig::default(),
             },
             o3_profile_config
@@ -3361,6 +3377,7 @@ model_verbosity = "high"
             tui_scroll_wheel_tick_detect_max_ms: None,
             tui_scroll_wheel_like_max_duration_ms: None,
             tui_scroll_invert: false,
+            tui_alternate_screen: AltScreenMode::Auto,
             otel: OtelConfig::default(),
         };
 
@@ -3461,6 +3478,7 @@ model_verbosity = "high"
             tui_scroll_wheel_tick_detect_max_ms: None,
             tui_scroll_wheel_like_max_duration_ms: None,
             tui_scroll_invert: false,
+            tui_alternate_screen: AltScreenMode::Auto,
             otel: OtelConfig::default(),
         };
 
@@ -3547,6 +3565,7 @@ model_verbosity = "high"
             tui_scroll_wheel_tick_detect_max_ms: None,
             tui_scroll_wheel_like_max_duration_ms: None,
             tui_scroll_invert: false,
+            tui_alternate_screen: AltScreenMode::Auto,
             otel: OtelConfig::default(),
         };