feat(core): auto-detect UTF-8 BOM for PowerShell scripts on Windows

- Add needsUtf8Bom() to detect when UTF-8 BOM is needed based on file
  extension and system code page
- PowerShell 5.1 on non-UTF-8 Windows systems (e.g. GBK) requires BOM
  to read scripts correctly
- Remove default UTF8 encoding; undefined now triggers auto-detection
- Add tests for needsUtf8Bom() covering Windows/non-Windows scenarios

This ensures PowerShell scripts are written with UTF-8 BOM on systems
that need it, fixing character encoding issues for non-ASCII content.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

packages/core/src/config/config.ts

@@ -37,7 +37,6 @@ import {
   type FileSystemService,
   StandardFileSystemService,
   type FileEncodingType,
-  FileEncoding,
 } from '../services/fileSystemService.js';
 import { GitService } from '../services/gitService.js';
 
@@ -523,7 +522,7 @@ export class Config {
   private readonly truncateToolOutputLines: number;
   private readonly eventEmitter?: EventEmitter;
   private readonly channel: string | undefined;
-  private readonly defaultFileEncoding: FileEncodingType;
+  private readonly defaultFileEncoding: FileEncodingType | undefined;
   private readonly enableHooks: boolean;
   private readonly hooks?: Record<string, unknown>;
   private readonly hooksConfig?: Record<string, unknown>;
@@ -641,7 +640,7 @@ export class Config {
     this.truncateToolOutputLines =
       params.truncateToolOutputLines ?? DEFAULT_TRUNCATE_TOOL_OUTPUT_LINES;
     this.channel = params.channel;
-    this.defaultFileEncoding = params.defaultFileEncoding ?? FileEncoding.UTF8;
+    this.defaultFileEncoding = params.defaultFileEncoding;
     this.storage = new Storage(this.targetDir);
     this.inputFormat = params.inputFormat ?? InputFormat.TEXT;
     this.fileExclusions = new FileExclusions(this);
@@ -1647,7 +1646,7 @@ export class Config {
    * Get the default file encoding for new files.
    * @returns FileEncodingType
    */
-  getDefaultFileEncoding(): FileEncodingType {
+  getDefaultFileEncoding(): FileEncodingType | undefined {
     return this.defaultFileEncoding;
   }