fix: show expert outputs before synthesis

Parallel expert runs previously exposed only child status, while the synthesized answer appeared above the expert cards. Each expert now retains and exposes its complete output, the cards can be expanded, and the final synthesis is rendered beneath them and persisted with the conversation.

Release note: 多专家并行分析现在可展开查看每位专家的完整输出，并在其下方显示总 Agent 的综合结果。

src/shared/contracts.ts

@@ -957,6 +957,7 @@ export const subagentEventSchema = z
       'cancelled'
     ]),
     reason: z.string().trim().min(1).max(240).optional(),
+    output: z.string().optional(),
     error: z.string().trim().min(1).max(1_000).optional()
   })
   .strict()