Remove resurrected /collab slash command (#22535)

## Summary
`/collab` was intentionally removed in
[#12012](https://github.com/openai/codex/pull/12012), but the
TUI/app-server migration accidentally brought that slash-command path
back. This restores the earlier product decision so the TUI no longer
advertises or dispatches `/collab`. This command was redundant because
it did the same thing as `/plan` but in a less-intuitive way.

## What Changed
- Remove `SlashCommand::Collab` from the TUI slash-command surface.
- Delete the picker and app-event plumbing that only existed to service
`/collab`.
- Remove obsolete TUI test coverage for the deleted picker flow.

codex-rs/tui/src/chatwidget/model_popups.rs

@@ -213,51 +213,6 @@ impl ChatWidget {
         });
     }
 
-    pub(crate) fn open_collaboration_modes_popup(&mut self) {
-        let presets = collaboration_modes::presets_for_tui(self.model_catalog.as_ref());
-        if presets.is_empty() {
-            self.add_info_message(
-                "No collaboration modes are available right now.".to_string(),
-                /*hint*/ None,
-            );
-            return;
-        }
-
-        let current_kind = self
-            .active_collaboration_mask
-            .as_ref()
-            .and_then(|mask| mask.mode)
-            .or_else(|| {
-                collaboration_modes::default_mask(self.model_catalog.as_ref())
-                    .and_then(|mask| mask.mode)
-            });
-        let items: Vec<SelectionItem> = presets
-            .into_iter()
-            .map(|mask| {
-                let name = mask.name.clone();
-                let is_current = current_kind == mask.mode;
-                let actions: Vec<SelectionAction> = vec![Box::new(move |tx| {
-                    tx.send(AppEvent::UpdateCollaborationMode(mask.clone()));
-                })];
-                SelectionItem {
-                    name,
-                    is_current,
-                    actions,
-                    dismiss_on_select: true,
-                    ..Default::default()
-                }
-            })
-            .collect();
-
-        self.bottom_pane.show_selection_view(SelectionViewParams {
-            title: Some("Select Collaboration Mode".to_string()),
-            subtitle: Some("Pick a collaboration preset.".to_string()),
-            footer_hint: Some(standard_popup_hint_line()),
-            items,
-            ..Default::default()
-        });
-    }
-
     fn model_selection_actions(
         model_for_action: String,
         effort_for_action: Option<ReasoningEffortConfig>,

codex-rs/tui/src/chatwidget/slash_dispatch.rs

@@ -238,16 +238,6 @@ impl ChatWidget {
                     );
                 }
             }
-            SlashCommand::Collab => {
-                if !self.collaboration_modes_enabled() {
-                    self.add_info_message(
-                        "Collaboration modes are disabled.".to_string(),
-                        Some("Enable collaboration modes to use /collab.".to_string()),
-                    );
-                    return;
-                }
-                self.open_collaboration_modes_popup();
-            }
             SlashCommand::Side => {
                 self.request_empty_side_conversation();
             }
@@ -965,7 +955,6 @@ impl ChatWidget {
             | SlashCommand::Personality
             | SlashCommand::Plan
             | SlashCommand::Goal
-            | SlashCommand::Collab
             | SlashCommand::Side
             | SlashCommand::Keymap
             | SlashCommand::Agent

codex-rs/tui/src/chatwidget/tests/plan_mode.rs

@@ -1367,63 +1367,6 @@ async fn mode_switch_surfaces_reasoning_change_notification_when_model_stays_sam
     );
 }
 
-#[tokio::test]
-async fn collab_slash_command_opens_picker_and_updates_mode() {
-    let (mut chat, mut rx, mut op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
-    chat.thread_id = Some(ThreadId::new());
-    chat.set_feature_enabled(Feature::CollaborationModes, /*enabled*/ true);
-
-    chat.dispatch_command(SlashCommand::Collab);
-    let popup = render_bottom_popup(&chat, /*width*/ 80);
-    assert!(
-        popup.contains("Select Collaboration Mode"),
-        "expected collaboration picker: {popup}"
-    );
-
-    chat.handle_key_event(KeyEvent::from(KeyCode::Enter));
-    let selected_mask = match rx.try_recv() {
-        Ok(AppEvent::UpdateCollaborationMode(mask)) => mask,
-        other => panic!("expected UpdateCollaborationMode event, got {other:?}"),
-    };
-    chat.set_collaboration_mask(selected_mask);
-
-    chat.bottom_pane
-        .set_composer_text("hello".to_string(), Vec::new(), Vec::new());
-    chat.handle_key_event(KeyEvent::from(KeyCode::Enter));
-    match next_submit_op(&mut op_rx) {
-        Op::UserTurn {
-            collaboration_mode:
-                Some(CollaborationMode {
-                    mode: ModeKind::Default,
-                    ..
-                }),
-            personality: Some(Personality::Pragmatic),
-            ..
-        } => {}
-        other => {
-            panic!("expected Op::UserTurn with code collab mode, got {other:?}")
-        }
-    }
-
-    chat.bottom_pane
-        .set_composer_text("follow up".to_string(), Vec::new(), Vec::new());
-    chat.handle_key_event(KeyEvent::from(KeyCode::Enter));
-    match next_submit_op(&mut op_rx) {
-        Op::UserTurn {
-            collaboration_mode:
-                Some(CollaborationMode {
-                    mode: ModeKind::Default,
-                    ..
-                }),
-            personality: Some(Personality::Pragmatic),
-            ..
-        } => {}
-        other => {
-            panic!("expected Op::UserTurn with code collab mode, got {other:?}")
-        }
-    }
-}
-
 #[tokio::test]
 async fn plan_slash_command_switches_to_plan_mode() {
     let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;