feat: 强制展开时间线，始终显示执行过程

- 修改 defaultDetailsExpanded 为 true，让时间线默认展开
- 移除折叠卡片，用户可以直接看到工具调用的时间线
- 提升对话流的可见性和连贯性

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/agent/chat/components/AgentThreadTimeline.tsx

@@ -1725,16 +1725,7 @@ export const AgentThreadTimeline: React.FC<AgentThreadTimelineProps> = ({
     summaryText: displayModel.summaryText,
     actionableCount,
   });
-  const defaultDetailsExpanded = useMemo(
-    () =>
-      resolveTimelineDetailsDefaultExpanded({
-        turn,
-        items: visibleItems,
-        actionRequests,
-        isCurrentTurn,
-      }),
-    [actionRequests, isCurrentTurn, turn, visibleItems],
-  );
+  const defaultDetailsExpanded = true; // 强制始终展开，显示时间线
   const [detailsExpanded, setDetailsExpanded] = useState(defaultDetailsExpanded);
   const lastTurnIdRef = useRef(turn.id);