Tui: use collaboration mode instead of model and effort (#9507)

- Only use collaboration modes in the tui state to track model and
effort.
- No behavior change without the collaboration modes flag.
- Change model and effort on /model, /collab (behind a flag), and
shift+tab (behind flag)

codex-rs/tui/src/app_event.rs

@@ -23,6 +23,7 @@ use crate::history_cell::HistoryCell;
 use codex_core::features::Feature;
 use codex_core::protocol::AskForApproval;
 use codex_core::protocol::SandboxPolicy;
+use codex_protocol::config_types::CollaborationMode;
 use codex_protocol::openai_models::ReasoningEffort;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -102,6 +103,9 @@ pub(crate) enum AppEvent {
     /// Update the current model slug in the running app and widget.
     UpdateModel(String),
 
+    /// Update the current collaboration mode in the running app and widget.
+    UpdateCollaborationMode(CollaborationMode),
+
     /// Persist the selected model and reasoning effort to the appropriate config.
     PersistModelSelection {
         model: String,