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/utils/fileReadCache.ts

@@ -1,9 +1,10 @@
 import { detectFileEncoding } from './file.js'
+import { type FileEncoding, decodeBuffer } from './encoding.js'
 import { getFsImplementation } from './fsOperations.js'
 
 type CachedFileData = {
   content: string
-  encoding: BufferEncoding
+  encoding: FileEncoding
   mtime: number
 }
 
@@ -19,7 +20,7 @@ class FileReadCache {
    * Reads a file with caching. Returns both content and encoding.
    * Cache key includes file path and modification time for automatic invalidation.
    */
-  readFile(filePath: string): { content: string; encoding: BufferEncoding } {
+  readFile(filePath: string): { content: string; encoding: FileEncoding } {
     const fs = getFsImplementation()
 
     // Get file stats for cache invalidation
@@ -45,9 +46,8 @@ class FileReadCache {
 
     // Cache miss or stale data - read the file
     const encoding = detectFileEncoding(filePath)
-    const content = fs
-      .readFileSync(filePath, { encoding })
-      .replaceAll('\r\n', '\n')
+    const rawBuffer = fs.readFileBytesSync(filePath)
+    const content = decodeBuffer(rawBuffer, encoding).replaceAll('\r\n', '\n')
 
     // Update cache
     this.cache.set(cacheKey, {