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/cli/src/config/config.ts

@@ -10,7 +10,6 @@ import {
   Config,
   DEFAULT_QWEN_EMBEDDING_MODEL,
   FileDiscoveryService,
-  FileEncoding,
   getAllGeminiMdFilenames,
   loadServerHierarchicalMemory,
   setGeminiMdFilename as setServerGeminiMdFilename,
@@ -1041,8 +1040,7 @@ export async function loadCliConfig(
     // always be true and the settings file can never disable recording.
     chatRecording:
       argv.chatRecording ?? settings.general?.chatRecording ?? true,
-    defaultFileEncoding:
-      settings.general?.defaultFileEncoding ?? FileEncoding.UTF8,
+    defaultFileEncoding: settings.general?.defaultFileEncoding,
     lsp: {
       enabled: lspEnabled,
     },