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/constants/tools.ts

@@ -121,7 +121,7 @@ export const COORDINATOR_MODE_ALLOWED_TOOLS = new Set([
  * Core tools that are always loaded with full schema at initialization.
  * These tools are never deferred — they appear in the initial prompt.
  * All other tools (non-core built-in + all MCP tools) are deferred
- * and must be discovered via ToolSearchTool / ExecuteTool.
+ * and must be discovered via ToolSearchTool / ExecuteExtraTool.
  */
 export const CORE_TOOLS = new Set([
   // File operations
@@ -162,6 +162,6 @@ export const CORE_TOOLS = new Set([
   SLEEP_TOOL_NAME, // 'Sleep'
   // Tool discovery (always loaded)
   TOOL_SEARCH_TOOL_NAME, // 'ToolSearch'
-  EXECUTE_TOOL_NAME, // 'ExecuteTool'
+  EXECUTE_TOOL_NAME, // 'ExecuteExtraTool'
   SYNTHETIC_OUTPUT_TOOL_NAME, // 'SyntheticOutput'
 ]) as ReadonlySet<string>