refactor: 统一自建 Tool Search — 移除 tool_reference/defer_loading 依赖，全 provider 通用

- 重命名 ExecuteTool → ExecuteExtraTool，作为一等工具始终可用
- ToolSearchTool 输出改为纯文本（区分 core/deferred），移除 tool_reference blocks
- 移除 modelSupportsToolReference() 及相关 GrowthBook 配置
- 移除 API 侧 defer_loading 字段和 tool search beta header 注入
- 简化 system prompt（工具使用指南从 ~120 行压缩到 ~10 行）
- extractDiscoveredToolNames 支持文本格式解析（向后兼容旧 session 的 tool_reference）
- 更新 promptEngineeringAudit 测试以匹配简化后的 prompt 结构

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/utils/messages.ts

@@ -3919,7 +3919,7 @@ Read the team config to discover your teammates' names. Check the task list peri
       )
       return wrapMessagesInSystemReminder([
         createUserMessage({
-          content: `The following tools were discovered as relevant to your task. Use ExecuteTool to invoke any of them by name:\n\n${lines.join('\n')}`,
+          content: `The following tools were discovered as relevant to your task. Use ExecuteExtraTool to invoke any of them by name:\n\n${lines.join('\n')}`,
           isMeta: true,
         }),
       ])