feat: 实现 Tool Search 基础设施层（CORE_TOOLS 白名单 + TF-IDF 索引 + ExecuteTool + 搜索增强）

- 新增 CORE_TOOLS 白名单常量（31 个核心工具），重构 isDeferredTool 为白名单制判定
- 新建 TF-IDF 工具索引模块（toolIndex.ts），复用 localSearch.ts 算法函数
- 新建 ExecuteTool 跨 API provider 统一工具执行入口
- 增强 ToolSearchTool：TF-IDF 搜索路径、discover: 模式、并行搜索合并、文本模式回退
- 新增 27 个单元测试，precheck 零错误通过（4108 tests pass）

Co-Authored-By: glm-5.1[1m] <zai-org@claude-code-best.win>

src/utils/attachments.ts

@@ -1,4 +1,5 @@
 // biome-ignore-all assist/source/organizeImports: ANT-ONLY import markers must not be reordered
+import type { ToolDiscoveryResult } from '../services/toolSearch/prefetch.js'
 import {
   logEvent,
   type AnalyticsMetadata_I_VERIFIED_THIS_IS_NOT_CODE_OR_FILEPATHS,
@@ -97,6 +98,12 @@ const skillSearchModules = feature('EXPERIMENTAL_SKILL_SEARCH')
         require('../services/skillSearch/prefetch.js') as typeof import('../services/skillSearch/prefetch.js'),
     }
   : null
+const toolSearchModules = feature('EXPERIMENTAL_TOOL_SEARCH')
+  ? {
+      prefetch:
+        require('../services/toolSearch/prefetch.js') as typeof import('../services/toolSearch/prefetch.js'),
+    }
+  : null
 const autoModeStateModule = feature('TRANSCRIPT_CLASSIFIER')
   ? (require('./permissions/autoModeState.js') as typeof import('./permissions/autoModeState.js'))
   : null
@@ -553,6 +560,14 @@ export type Attachment =
         activePath?: string
       }
     }
+  | {
+      type: 'tool_discovery'
+      tools: ToolDiscoveryResult[]
+      trigger: 'assistant_turn' | 'user_input'
+      queryText: string
+      durationMs: number
+      indexSize: number
+    }
   | {
       type: 'queued_command'
       prompt: string | Array<ContentBlockParam>
@@ -830,6 +845,25 @@ export async function getAttachments(
               }),
             ]
           : []),
+        // Tool discovery on turn 0. Inter-turn discovery runs via
+        // startToolSearchPrefetch in query.ts.
+        ...(feature('EXPERIMENTAL_TOOL_SEARCH') &&
+        toolSearchModules &&
+        !options?.skipSkillDiscovery
+          ? [
+              maybe('tool_discovery', async () => {
+                if (suppressNextDiscovery) {
+                  return []
+                }
+                const result =
+                  await toolSearchModules.prefetch.getTurnZeroToolSearchPrefetch(
+                    input,
+                    context.options.tools ?? [],
+                  )
+                return result ? [result] : []
+              }),
+            ]
+          : []),
       ]
     : []