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/tui2/src/lib.rs

@@ -22,6 +22,8 @@ use codex_core::config::resolve_oss_provider;
 use codex_core::find_thread_path_by_id_str;
 use codex_core::get_platform_sandbox;
 use codex_core::protocol::AskForApproval;
+use codex_core::terminal::Multiplexer;
+use codex_protocol::config_types::AltScreenMode;
 use codex_protocol::config_types::SandboxMode;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use std::fs::OpenOptions;
@@ -515,12 +517,39 @@ async fn run_ratatui_app(
         resume_picker::ResumeSelection::StartFresh
     };
 
-    let Cli { prompt, images, .. } = cli;
+    let Cli {
+        prompt,
+        images,
+        no_alt_screen,
+        ..
+    } = cli;
 
     // Run the main chat + transcript UI on the terminal's alternate screen so
     // the entire viewport can be used without polluting normal scrollback. This
     // mirrors the behavior of the legacy TUI but keeps inline mode available
     // for smaller prompts like onboarding and model migration.
+    //
+    // However, alternate screen prevents scrollback in terminal multiplexers like
+    // Zellij that strictly follow the xterm spec (which disallows scrollback in
+    // alternate screen buffers). This auto-detects the terminal and disables
+    // alternate screen in Zellij while keeping it enabled elsewhere.
+    let use_alt_screen = if no_alt_screen {
+        // CLI flag explicitly disables alternate screen
+        false
+    } else {
+        match config.tui_alternate_screen {
+            AltScreenMode::Always => true,
+            AltScreenMode::Never => false,
+            AltScreenMode::Auto => {
+                // Auto-detect: disable in Zellij, enable elsewhere
+                let terminal_info = codex_core::terminal::terminal_info();
+                !matches!(terminal_info.multiplexer, Some(Multiplexer::Zellij { .. }))
+            }
+        }
+    };
+
+    // Set flag on Tui so all enter_alt_screen() calls respect the setting
+    tui.set_alt_screen_enabled(use_alt_screen);
     let _ = tui.enter_alt_screen();
 
     let app_result = App::run(