feat: Preserve UTF-8 BOM when editing files (Fix #1672)

- Add FileEncoding constants (UTF8, UTF8_BOM)
- Add detectFileBOM() to detect existing file encoding
- Modify writeTextFile() to support BOM option
- Add defaultFileEncoding configuration option
- Preserve BOM when editing existing files
- Use configured encoding for new files
- Add comprehensive tests (unit, integration, e2e)
- Update documentation

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

packages/cli/src/config/config.ts

@@ -10,6 +10,7 @@ import {
   Config,
   DEFAULT_QWEN_EMBEDDING_MODEL,
   FileDiscoveryService,
+  FileEncoding,
   getCurrentGeminiMdFilename,
   loadServerHierarchicalMemory,
   setGeminiMdFilename as setServerGeminiMdFilename,
@@ -1030,6 +1031,8 @@ 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,
     lsp: {
       enabled: lspEnabled,
     },