fix: preserve original encoding when reading/writing non-UTF-8 files

Fixes #2069

- Add iconv-lite dependency for non-UTF-8 encoding support
- Add iconvHelper.ts as a CJS/ESM compatibility wrapper
- Update readFileWithEncoding() to detect and handle GBK/Big5/Shift_JIS
  using BOM detection -> UTF-8 validation -> chardet -> iconv-lite fallback
- Add detectFileEncoding() to identify file encoding before writes
- Update writeTextFile() to accept encoding option and encode with iconv-lite
- Update WriteFileTool and EditTool to detect and preserve original encoding
- Add tests for GBK read/write, detectFileEncoding, and encoding option

packages/cli/src/acp-integration/service/filesystem.ts

@@ -57,7 +57,7 @@ export class AcpFileSystemService implements FileSystemService {
   async writeTextFile(
     filePath: string,
     content: string,
-    options?: { bom?: boolean },
+    options?: { bom?: boolean; encoding?: string },
   ): Promise<void> {
     if (!this.capabilities.writeTextFile) {
       return this.fallback.writeTextFile(filePath, content, options);