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>

packages/builtin-tools/src/tools/BashTool/BashTool.tsx

@@ -29,6 +29,7 @@ import { extractClaudeCodeHints } from 'src/utils/claudeCodeHints.js';
 import { detectCodeIndexingFromCommand } from 'src/utils/codeIndexing.js';
 import { isEnvTruthy } from 'src/utils/envUtils.js';
 import { isENOENT, ShellError } from 'src/utils/errors.js';
+import { decodeBuffer } from 'src/utils/encoding.js';
 import { detectFileEncoding, detectLineEndings, getFileModificationTime, writeTextContent } from 'src/utils/file.js';
 import { fileHistoryEnabled, fileHistoryTrackEdit } from 'src/utils/fileHistory.js';
 import { truncate } from 'src/utils/format.js';
@@ -511,7 +512,8 @@ async function applySedEdit(
   const encoding = detectFileEncoding(absoluteFilePath);
   let originalContent: string;
   try {
-    originalContent = await fs.readFile(absoluteFilePath, { encoding });
+    const rawBuffer = await fs.readFileBytes(absoluteFilePath);
+    originalContent = decodeBuffer(rawBuffer, encoding);
   } catch (e) {
     if (isENOENT(e)) {
       return {