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/arena/ArenaStatusDialog.tsx

@@ -12,7 +12,7 @@ import {
   type ArenaAgentState,
   type InProcessBackend,
   type AgentStatsSummary,
-  isTerminalStatus,
+  isSettledStatus,
   ArenaSessionStatus,
   DISPLAY_MODE,
 } from '@qwen-code/qwen-code-core';
@@ -46,7 +46,7 @@ function pad(
 }
 
 function getElapsedMs(agent: ArenaAgentState): number {
-  if (isTerminalStatus(agent.status)) {
+  if (isSettledStatus(agent.status)) {
     return agent.stats.durationMs;
   }
   return Date.now() - agent.startedAt;
@@ -61,6 +61,8 @@ function getSessionStatusLabel(status: ArenaSessionStatus): {
       return { text: 'Running', color: theme.status.success };
     case ArenaSessionStatus.INITIALIZING:
       return { text: 'Initializing', color: theme.status.warning };
+    case ArenaSessionStatus.IDLE:
+      return { text: 'Idle', color: theme.status.success };
     case ArenaSessionStatus.COMPLETED:
       return { text: 'Completed', color: theme.status.success };
     case ArenaSessionStatus.CANCELLED: