feat(subagent): 实验功能设置新增子代理模型配置

- 子代理设置页支持选择次主力模型，仅保存模型引用（不再写补丁字段），
  子代理直接继承所选模型的上下文长度与思考模式
- 界面只读展示继承的上下文长度与思考模式，移除补丁编辑项
- 仪表盘统计识别子代理请求（__secondary__）：归为「子代理模型（估算）」
  并缓存次主力模型定价；自定义网关模型价格按官方同名模型跨 provider 匹配
- bump v0.7.1

src/types/dashboard.ts

@@ -24,6 +24,8 @@ export interface ModelRow extends TotalsRow {
   modelResolved: string;
   priceId: string;
   costEstimated: boolean;
+  /** True when any record in this row came from a subagent (secondary-model) request. */
+  isSecondary: boolean;
 }
 
 export interface RecentRow {
@@ -40,6 +42,8 @@ export interface RecentRow {
   costEstimated: boolean;
   priceId: string;
   fromEnv: boolean;
+  /** True when this request came from a subagent (secondary-model) call. */
+  isSecondary: boolean;
 }
 
 export interface RangeStats {