feat(arena): add IDLE status for agent follow-up task support

- Introduce AgentStatus.IDLE for agents that finished work but can accept follow-up messages
- Add isSettledStatus() helper to check if agent is settled (IDLE or terminal)
- Update ArenaManager to transition to IDLE after agents finish initial task
- Keep agent tabs visible when session is IDLE so users can continue interacting
- Fix listener cleanup to not detach on IDLE (agents remain alive)
- Update tests to expect 'idle' status after successful completion

This enables the arena collaboration feature where agents can receive
additional tasks after completing their initial work.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

packages/cli/src/ui/components/Composer.tsx

@@ -104,8 +104,8 @@ export const Composer = () => {
 
       {/* Exclusive area: only one component visible at a time */}
       {/* Hide footer when a confirmation dialog (e.g. ask_user_question) is active */}
-      {!showSuggestions &&
-        uiState.streamingState !== StreamingState.WaitingForConfirmation &&
+      {uiState.isInputActive &&
+        !showSuggestions &&
         (showShortcuts ? (
           <KeyboardShortcuts />
         ) : (