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,
     },

packages/cli/src/config/settingsSchema.ts

@@ -244,6 +244,20 @@ const SETTINGS_SCHEMA = {
           'Enable saving chat history to disk. Disabling this will also prevent --continue and --resume from working.',
         showInDialog: false,
       },
+      defaultFileEncoding: {
+        type: 'enum',
+        label: 'Default File Encoding',
+        category: 'General',
+        requiresRestart: false,
+        default: 'utf-8',
+        description:
+          'Default encoding for new files. Use "utf-8" (default) for UTF-8 without BOM, or "utf-8-bom" for UTF-8 with BOM. Only change this if your project specifically requires BOM.',
+        showInDialog: false,
+        options: [
+          { value: 'utf-8', label: 'UTF-8 (without BOM)' },
+          { value: 'utf-8-bom', label: 'UTF-8 with BOM' },
+        ],
+      },
     },
   },
   output: {