feat: 添加 web_search 联网搜索工具 (21个工具)

- 新增 web_search 工具：AI 可主动联网搜索获取实时信息
- 搜索引擎：DuckDuckGo 优先，Bing 作为备用
- 返回结构化结果：标题、URL、摘要
- 自动启用，无需用户确认（只读操作）
- 更新 README 工具列表和计数

src/main/ipc.ts

@@ -24,6 +24,7 @@ import {
   handleMoveFile,
   handleCopyFile,
   handleWebFetch,
+  handleWebSearch,
   handleAppendFile,
   handleEditFile,
   handleGetFileInfo,
@@ -50,6 +51,7 @@ function summarizeResult(toolName: string, result: Record<string, unknown>): str
     case 'move_file': return `${result.source} → ${result.destination}`;
     case 'copy_file': return `${result.source} → ${result.destination}`;
     case 'web_fetch': return `${result.status} | ${result.length} chars`;
+    case 'web_search': return `"${result.query}" → ${result.total} 条结果`;
     case 'append_file': return `${result.path} (${result.newSize}B)`;
     case 'edit_file': return `${result.path} (${result.replaceCount} 处替换)`;
     case 'get_file_info': return `${result.name} (${result.type}, ${result.size}B)`;
@@ -151,6 +153,7 @@ export function setupIPC(): void {
         case 'move_file':          result = await handleMoveFile(args as { source: string; destination: string }); break;
         case 'copy_file':          result = await handleCopyFile(args as { source: string; destination: string; recursive?: boolean }); break;
         case 'web_fetch':          result = await handleWebFetch(args as { url: string; max_chars?: number; extract_mode?: string }); break;
+        case 'web_search':         result = await handleWebSearch(args as { query: string; max_results?: number }); break;
         case 'append_file':        result = await handleAppendFile(args as { path: string; content: string; newline?: boolean }); break;
         case 'edit_file':          result = await handleEditFile(args as { path: string; old_text: string; new_text: string; all?: boolean }); break;
         case 'get_file_info':      result = await handleGetFileInfo(args as { path: string }); break;