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/utils/displayUtils.ts

@@ -17,6 +17,8 @@ export interface StatusLabel {
 
 export function getArenaStatusLabel(status: AgentStatus): StatusLabel {
   switch (status) {
+    case AgentStatus.IDLE:
+      return { icon: '✓', text: 'Idle', color: theme.status.success };
     case AgentStatus.COMPLETED:
       return { icon: '✓', text: 'Done', color: theme.status.success };
     case AgentStatus.CANCELLED: