Move terminal module to terminal-detection crate (#15216)

- Move core/src/terminal.rs and its tests into a standalone
terminal-detection workspace crate.
- Update direct consumers to depend on codex-terminal-detection and
import terminal APIs directly.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/tui/src/lib.rs

@@ -33,7 +33,6 @@ use codex_core::format_exec_policy_error_with_source;
 use codex_core::path_utils;
 use codex_core::read_session_meta_line;
 use codex_core::state_db::get_state_db;
-use codex_core::terminal::Multiplexer;
 use codex_core::windows_sandbox::WindowsSandboxLevelExt;
 use codex_protocol::ThreadId;
 use codex_protocol::config_types::AltScreenMode;
@@ -43,6 +42,8 @@ use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::RolloutItem;
 use codex_protocol::protocol::RolloutLine;
 use codex_state::log_db;
+use codex_terminal_detection::Multiplexer;
+use codex_terminal_detection::terminal_info;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use codex_utils_oss::ensure_oss_provider_ready;
 use codex_utils_oss::get_default_model_for_oss_provider;
@@ -1138,7 +1139,7 @@ fn determine_alt_screen_mode(no_alt_screen: bool, tui_alternate_screen: AltScree
             AltScreenMode::Always => true,
             AltScreenMode::Never => false,
             AltScreenMode::Auto => {
-                let terminal_info = codex_core::terminal::terminal_info();
+                let terminal_info = terminal_info();
                 !matches!(terminal_info.multiplexer, Some(Multiplexer::Zellij { .. }))
             }
         }