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/layouts/DefaultAppLayout.tsx

@@ -67,8 +67,8 @@ export const DefaultAppLayout: React.FC = () => {
         <ExitWarning />
       </Box>
 
-      {/* Tab bar: visible whenever in-process agents exist */}
-      {hasAgents && <AgentTabBar />}
+      {/* Tab bar: visible whenever in-process agents exist and input is active */}
+      {hasAgents && !uiState.dialogsVisible && <AgentTabBar />}
     </Box>
   );
 };