feat: 子会话结果增加创建/结束时间戳

- SubSessionResult 新增 CreatedAt / FinishedAt 字段
- manager.Dispatch 在各路径统一设置时间戳：成功/失败/超时

Co-Authored-By: Claude <noreply@anthropic.com>

backend/ai-core/internal/subsession/manager.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"git.yeij.top/AskaEth/Cyrene/pkg/logger"
 	"sync"
+	"time"
 
 	"git.yeij.top/AskaEth/Cyrene/ai-core/internal/bus"
 	"git.yeij.top/AskaEth/Cyrene/ai-core/internal/llm"
@@ -109,7 +110,8 @@ func (m *Manager) Dispatch(
 				}
 			}()
 
-			result := model.SubSessionResult{Type: p.Type()}
+			createdAt := time.Now()
+			result := model.SubSessionResult{Type: p.Type(), CreatedAt: createdAt}
 				m.getBus().Publish(bus.BusEvent{
 					Type:    bus.EventSubSessionStarted,
 					Payload: bus.SubSessionPayload{SubType: p.Type(), Status: "started"},
@@ -125,7 +127,8 @@ func (m *Manager) Dispatch(
 			if err != nil {
 				result.Error = fmt.Sprintf("创建上下文失败: %v", err)
 				logger.Printf("[subsession] %s 创建上下文失败: %v", p.Type(), err)
-				resultCh <- result
+				result.FinishedAt = time.Now()
+			resultCh <- result
 				return
 			}
 
@@ -134,6 +137,7 @@ func (m *Manager) Dispatch(
 			// 执行子会话
 			subResult, execErr := p.Execute(subCtx, llmMessages)
 			if execErr != nil {
+				result.FinishedAt = time.Now()
 				result.Error = fmt.Sprintf("执行失败: %v", execErr)
 				logger.Printf("[subsession] %s 执行失败: %v", p.Type(), execErr)
 				resultCh <- result
@@ -143,12 +147,14 @@ func (m *Manager) Dispatch(
 			// 检查超时
 			select {
 			case <-subCtx.Done():
+				result.FinishedAt = time.Now()
 				result.Error = "子会话超时"
 				logger.Printf("[subsession] %s 超时 (limit=%v)", p.Type(), p.Timeout())
 			default:
 				if subResult != nil {
 					result = *subResult
 					result.Type = p.Type()
+					result.CreatedAt = createdAt
 					logger.Printf("[subsession] %s 完成: 摘要=%s", p.Type(), truncate(result.Summary, 50))
 				}
 			}