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/tui/src/cli.rs

@@ -85,6 +85,14 @@ pub struct Cli {
     #[arg(long = "add-dir", value_name = "DIR", value_hint = ValueHint::DirPath)]
     pub add_dir: Vec<PathBuf>,
 
+    /// Disable alternate screen mode
+    ///
+    /// Runs the TUI in inline mode, preserving terminal scrollback history. This is useful
+    /// in terminal multiplexers like Zellij that follow the xterm spec strictly and disable
+    /// scrollback in alternate screen buffers.
+    #[arg(long = "no-alt-screen", default_value_t = false)]
+    pub no_alt_screen: bool,
+
     #[clap(skip)]
     pub config_overrides: CliConfigOverrides,
 }