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