Terminal Detection Metadata for Per-Terminal Scroll Scaling (#8252)

# Terminal Detection Metadata for Per-Terminal Scroll Scaling

## Summary
Expand terminal detection into structured metadata (`TerminalInfo`) with
multiplexer awareness, plus a testable environment shim and
characterization tests.

## Context / Motivation
- TUI2 owns its viewport and scrolling model (see
`codex-rs/tui2/docs/tui_viewport_and_history.md`), so scroll behavior
must be consistent across terminals and independent of terminal
scrollback quirks.
- Prior investigations show mouse wheel scroll deltas vary noticeably by
terminal. To tune scroll scaling (line increments per wheel tick) we
need reliable terminal identification, including when running inside
tmux/zellij.
- tmux is especially tricky because it can mask the underlying terminal;
we now consult `tmux display-message` client termtype/name to attribute
sessions to the actual terminal rather than tmux itself.
- This remains backwards compatible with the existing OpenTelemetry
user-agent token because `user_agent()` is still derived from the same
environment signals (now via `TerminalInfo`).

## Changes
- Introduce `TerminalInfo`, `TerminalName`, and `Multiplexer` with
`TERM_PROGRAM`/`TERM`/multiplexer detection and user-agent formatting in
`codex-rs/core/src/terminal.rs`.
- Add an injectable `Environment` trait + `FakeEnvironment` for testing,
and comprehensive characterization tests covering known terminals, tmux
client termtype/name, and zellij.
- Document module usage and detection order; update `terminal_info()` to
be the primary interface for callers.

## Testing
- `cargo test -p codex-core terminal::tests`
- manually checked ghostty, iTerm2, Terminal.app, vscode, tmux, zellij,
Warp, alacritty, kitty.
```
2025-12-18T07:07:49.191421Z  INFO Detected terminal info terminal=TerminalInfo { name: Iterm2, term_program: Some("iTerm.app"), version: Some("3.6.6"), term: None, multiplexer: None }
2025-12-18T07:07:57.991776Z  INFO Detected terminal info terminal=TerminalInfo { name: AppleTerminal, term_program: Some("Apple_Terminal"), version: Some("455.1"), term: None, multiplexer: None }
2025-12-18T07:08:07.732095Z  INFO Detected terminal info terminal=TerminalInfo { name: WarpTerminal, term_program: Some("WarpTerminal"), version: Some("v0.2025.12.10.08.12.stable_03"), term: None, multiplexer: None }
2025-12-18T07:08:24.860316Z  INFO Detected terminal info terminal=TerminalInfo { name: Kitty, term_program: None, version: None, term: None, multiplexer: None }
2025-12-18T07:08:38.302761Z  INFO Detected terminal info terminal=TerminalInfo { name: Alacritty, term_program: None, version: None, term: None, multiplexer: None }
2025-12-18T07:08:50.887748Z  INFO Detected terminal info terminal=TerminalInfo { name: VsCode, term_program: Some("vscode"), version: Some("1.107.1"), term: None, multiplexer: None }
2025-12-18T07:10:01.309802Z  INFO Detected terminal info terminal=TerminalInfo { name: WezTerm, term_program: Some("WezTerm"), version: Some("20240203-110809-5046fc22"), term: None, multiplexer: None }
2025-12-18T08:05:17.009271Z  INFO Detected terminal info terminal=TerminalInfo { name: Ghostty, term_program: Some("ghostty"), version: Some("1.2.3"), term: None, multiplexer: None }
2025-12-18T08:05:23.819973Z  INFO Detected terminal info terminal=TerminalInfo { name: Ghostty, term_program: Some("ghostty"), version: Some("1.2.3"), term: Some("xterm-ghostty"), multiplexer: Some(Tmux { version: Some("3.6a") }) }
2025-12-18T08:05:35.572853Z  INFO Detected terminal info terminal=TerminalInfo { name: Ghostty, term_program: Some("ghostty"), version: Some("1.2.3"), term: None, multiplexer: Some(Zellij) }
```

## Notes / Follow-ups
- Next step is to wire `TerminalInfo` into TUI2’s scroll scaling
configuration and add a per-terminal tuning table.
- The log output in TUI2 helps validate real-world detection before
applying behavior changes.

.codespellignore

@@ -1,2 +1,3 @@
 iTerm
+iTerm2
 psuedo

.codespellrc

@@ -3,4 +3,4 @@
 skip = .git*,vendor,*-lock.yaml,*.lock,.codespellrc,*test.ts,*.jsonl,frame*.txt
 check-hidden = true
 ignore-regex = ^\s*"image/\S+": ".*|\b(afterAll)\b
-ignore-words-list = ratatui,ser
+ignore-words-list = ratatui,ser,iTerm,iterm2,iterm

codex-rs/tui2/src/lib.rs

@@ -319,6 +319,9 @@ pub async fn run_main(
         .with(otel_logger_layer)
         .try_init();
 
+    let terminal_info = codex_core::terminal::terminal_info();
+    tracing::info!(terminal = ?terminal_info, "Detected terminal info");
+
     run_ratatui_app(
         cli,
         config,