feat: 添加 GBK 编码自动检测支持，文件读写工具透明处理非 UTF-8 文件

新增 encoding.ts 核心模块实现三层编码检测（BOM → UTF-8 fatal → GBK 回退），
改造同步/异步读取路径和写入路径，使 FileReadTool/FileEditTool/FileWriteTool
能正确处理 GBK 编码文件。包含完整单元测试和 spec 文档。

Co-Authored-By: glm-5-turbo <zai-org@claude-code-best.win>

src/components/permissions/SedEditPermissionRequest/SedEditPermissionRequest.tsx

@@ -3,6 +3,7 @@ import React, { Suspense, use, useMemo } from 'react';
 import { FileEditToolDiff } from 'src/components/FileEditToolDiff.js';
 import { getCwd } from 'src/utils/cwd.js';
 import { isENOENT } from 'src/utils/errors.js';
+import { decodeBuffer } from 'src/utils/encoding.js';
 import { detectEncodingForResolvedPath } from 'src/utils/fileRead.js';
 import { getFsImplementation } from 'src/utils/fsOperations.js';
 import { Text } from '@anthropic/ink';
@@ -33,9 +34,10 @@ export function SedEditPermissionRequest({ sedInfo, ...props }: SedEditPermissio
         // render correctly. This matches what readFileSync did before the
         // async conversion.
         const encoding = detectEncodingForResolvedPath(filePath);
-        const raw = await getFsImplementation().readFile(filePath, { encoding });
+        const rawBuffer = await getFsImplementation().readFileBytes(filePath);
+        const raw = decodeBuffer(rawBuffer, encoding).replaceAll('\r\n', '\n');
         return {
-          oldContent: raw.replaceAll('\r\n', '\n'),
+          oldContent: raw,
           fileExists: true,
         };
       })().catch((e: unknown): FileReadResult => {