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/fileRead.ts

@@ -13,39 +13,24 @@
  */
 
 import { logForDebugging } from './debug.js'
+import { type FileEncoding, decodeBuffer, detectEncoding } from './encoding.js'
 import { getFsImplementation, safeResolvePath } from './fsOperations.js'
 
 export type LineEndingType = 'CRLF' | 'LF'
 
 export function detectEncodingForResolvedPath(
   resolvedPath: string,
-): BufferEncoding {
+): FileEncoding {
   const { buffer, bytesRead } = getFsImplementation().readSync(resolvedPath, {
     length: 4096,
   })
 
-  // Empty files should default to utf8, not ascii
-  // This fixes a bug where writing emojis/CJK to empty files caused corruption
+  // Empty files default to utf8 — nothing to detect
   if (bytesRead === 0) {
     return 'utf8'
   }
 
-  if (bytesRead >= 2) {
-    if (buffer[0] === 0xff && buffer[1] === 0xfe) return 'utf16le'
-  }
-
-  if (
-    bytesRead >= 3 &&
-    buffer[0] === 0xef &&
-    buffer[1] === 0xbb &&
-    buffer[2] === 0xbf
-  ) {
-    return 'utf8'
-  }
-
-  // For non-empty files, default to utf8 since it's a superset of ascii
-  // and handles all Unicode characters properly
-  return 'utf8'
+  return detectEncoding(buffer.subarray(0, bytesRead))
 }
 
 export function detectLineEndingsForString(content: string): LineEndingType {
@@ -74,7 +59,7 @@ export function detectLineEndingsForString(content: string): LineEndingType {
  */
 export function readFileSyncWithMetadata(filePath: string): {
   content: string
-  encoding: BufferEncoding
+  encoding: FileEncoding
   lineEndings: LineEndingType
 } {
   const fs = getFsImplementation()
@@ -85,10 +70,10 @@ export function readFileSyncWithMetadata(filePath: string): {
   }
 
   const encoding = detectEncodingForResolvedPath(resolvedPath)
-  const raw = fs.readFileSync(resolvedPath, { encoding })
-  // Detect line endings from the raw head before CRLF normalization erases
-  // the distinction. 4096 code units is ≥ detectLineEndings's 4096-byte
-  // readSync sample (line endings are ASCII, so the unit mismatch is moot).
+  // Read raw Buffer first — readFileSync encoding option only accepts
+  // BufferEncoding, not gbk etc.
+  const rawBuffer = fs.readFileBytesSync(resolvedPath)
+  const raw = decodeBuffer(rawBuffer, encoding)
   const lineEndings = detectLineEndingsForString(raw.slice(0, 4096))
   return {
     content: raw.replaceAll('\r\n', '\n'),