feat(tui): remove Zellij TUI workarounds (#22214)

## Why

We added Zellij-specific TUI workarounds because older Zellij behavior
did not work with Codex's normal terminal model:

- #8555 made `tui.alternate_screen = "auto"` disable alternate screen in
Zellij so transcript history stayed available.
- #16578 avoided scroll-region operations in Zellij by emitting raw
newlines and using a separate composer styling path.

This PR removes both workarounds because the latest Zellij release
tested locally (`zellij 0.44.1`) works correctly with Codex's standard
TUI behavior: normal alternate-screen handling, redraw, and history
insertion.

## What Changed

- Removed the `InsertHistoryMode::Zellij` path and the Zellij-only
newline scrollback insertion behavior.
- Removed cached `is_zellij` state from the TUI and composer.
- Removed Zellij-specific composer styling, the helper snapshot, and the
`TerminalInfo::is_zellij()` convenience method that only served this
workaround.
- Changed `tui.alternate_screen = "auto"` to use alternate screen for
Zellij too; `--no-alt-screen` and `tui.alternate_screen = "never"` still
preserve the inline mode escape hatch.
- Updated the generated config schema description for
`tui.alternate_screen`.

## How to Test

Manual smoke path used with `zellij 0.44.1`:

1. Build and run this branch inside a Zellij `0.44.1` session with
default config.
2. Start Codex normally and produce enough assistant/tool output to
create scrollback.
3. Confirm the transcript remains readable, the composer renders
normally, and scrolling through terminal history works.
4. Resize the Zellij pane while output exists and confirm the TUI
redraws without duplicated, missing, or stale rows.
5. Compare with `--no-alt-screen` or `-c tui.alternate_screen=never` if
you want to verify the inline fallback still works.

Targeted tests:
- `just write-config-schema`
- `just fmt`
- `just fix -p codex-tui`
- `cargo test -p codex-terminal-detection`
- `cargo test -p codex-tui alternate_screen_auto_uses_alt_screen`

Attempted but did not complete locally:
- `cargo test -p codex-tui` built and ran the new test successfully,
then failed later on unrelated local failures in
`status_permissions_full_disk_managed_*` and a stack overflow in
`tests::fork_last_filters_latest_session_by_cwd_unless_show_all`.

## Documentation

No developers.openai.com Codex documentation update is needed for this
revert.

codex-rs/protocol/src/config_types.rs

@@ -465,22 +465,9 @@ pub enum TrustLevel {
 
 /// Controls whether the TUI uses the terminal's alternate screen buffer.
 ///
-/// **Background:** The alternate screen buffer provides a cleaner fullscreen experience
-/// without polluting the terminal's scrollback history. However, it conflicts with terminal
-/// multiplexers like Zellij that strictly follow the xterm specification, which defines
-/// that alternate screen buffers should not have scrollback.
-///
-/// **Zellij's behavior:** Zellij intentionally disables scrollback in alternate screen mode
-/// (see https://github.com/zellij-org/zellij/pull/1032) to comply with the xterm spec. This
-/// is by design and not configurable in Zellij—there is no option to enable scrollback in
-/// alternate screen mode.
-///
-/// **Solution:** This setting provides a pragmatic workaround:
-/// - `auto` (default): Automatically detect the terminal multiplexer. If running in Zellij,
-///   disable alternate screen to preserve scrollback. Enable it everywhere else.
-/// - `always`: Always use alternate screen mode (original behavior before this fix).
-/// - `never`: Never use alternate screen mode. Runs in inline mode, preserving scrollback
-///   in all multiplexers.
+/// - `auto` (default): Use alternate screen mode.
+/// - `always`: Always use alternate screen mode.
+/// - `never`: Never use alternate screen mode. Runs in inline mode, preserving scrollback.
 ///
 /// The CLI flag `--no-alt-screen` can override this setting at runtime.
 #[derive(
@@ -489,10 +476,10 @@ pub enum TrustLevel {
 #[serde(rename_all = "lowercase")]
 #[strum(serialize_all = "lowercase")]
 pub enum AltScreenMode {
-    /// Auto-detect: disable alternate screen in Zellij, enable elsewhere.
+    /// Use alternate screen mode.
     #[default]
     Auto,
-    /// Always use alternate screen (original behavior).
+    /// Always use alternate screen mode.
     Always,
     /// Never use alternate screen (inline mode only).
     Never,