feat: 统一智能体路由 + 删除 onboarding

- 新增 Unified 智能体（合并 health/diet/medication/exercise/default 全部工具）
- AI 自动判断意图并调用对应工具，前端胶囊自动切换
- 删除 onboarding 智能体及所有建档引导 UI
- 前端始终发 unified 类型，通过 tool_result 事件同步胶囊状态

backend/src/Health.WebApi/Endpoints/ai_chat_endpoints.cs

@@ -337,7 +337,13 @@ public static class AiChatEndpoints
         AgentType.Consultation => ConsultationAgentHandler.Tools,
         AgentType.Report => ReportAgentHandler.Tools,
         AgentType.Exercise => ExerciseAgentHandler.Tools,
-        AgentType.Onboarding => [CommonAgentHandler.ManageArchiveTool, CommonAgentHandler.CheckArchiveTool],
+        AgentType.Unified => [
+            ..HealthDataAgentHandler.Tools,
+            ..DietAgentHandler.Tools,
+            ..MedicationAgentHandler.Tools,
+            ..ExerciseAgentHandler.Tools,
+            ..CommonAgentHandler.Tools,
+        ],
         _ => CommonAgentHandler.Tools,
     };