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/slash_command.rs

@@ -54,6 +54,7 @@ pub enum SlashCommand {
     Realtime,
     Settings,
     TestApproval,
+    #[strum(serialize = "subagents")]
     MultiAgents,
     // Debugging commands.
     #[strum(serialize = "debug-m-drop")]