Use the shared implementer session constructor for forks

internal/agents/fork.go

@@ -186,24 +186,14 @@ func (m *Manager) fork(ctx context.Context, src api.Task, project api.Project, n
 		return api.Task{}, "", err
 	}
 
-	id := ids.New(ids.Session)
-	err = m.store.CreateSession(ctx, api.AgentSession{
-		Id:              id,
-		TaskId:          t.Id,
-		Vendor:          srcSess.Vendor,
-		Model:           srcSess.Model,
-		VendorSessionId: initialVendorSessionID(srcSess.Vendor),
-		State:           api.AgentSessionStateIdle,
-		Role:            api.Implementer,
-		CreatedBy:       creator.Login,
-		CreatedAt:       time.Now().UTC(),
-		ForkedFrom:      &srcSess.VendorSessionId,
-		ForkedAt:        &at,
-	}, t.ProjectId)
+	sess := sessionFor(t, creator, "", srcSess.Model)
+	sess.ForkedFrom = &srcSess.VendorSessionId
+	sess.ForkedAt = &at
+	err = m.store.CreateSession(ctx, sess, t.ProjectId)
 	if err != nil {
 		return api.Task{}, "", fmt.Errorf("task %s created, but its agent session was not: %w", t.Id, err)
 	}
-	return t, id, nil
+	return t, sess.Id, nil
 }
 
 func forkPoint(events []api.Event, sessionID string, seq int64) string {