Use subagents naming in the TUI (#14618)

- rename user-facing TUI multi-agent wording to subagents
- rename the surfaced slash command to `subagents` and update
tests/snapshots

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

codex-rs/tui/src/app.rs

@@ -1437,7 +1437,7 @@ impl App {
             .collect();
 
         self.chat_widget.show_selection_view(SelectionViewParams {
-            title: Some("Multi-agents".to_string()),
+            title: Some("Subagents".to_string()),
             subtitle: Some(AgentNavigationState::picker_subtitle()),
             footer_hint: Some(standard_popup_hint_line()),
             items,
@@ -5093,7 +5093,7 @@ mod tests {
             .map(|line| line.to_string())
             .collect::<Vec<_>>()
             .join("\n");
-        assert!(rendered.contains("Multi-agent will be enabled in the next session."));
+        assert!(rendered.contains("Subagents will be enabled in the next session."));
         Ok(())
     }