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
a5eb1733fa