fix(agents): preserve YAML frontmatter fields when saving agent settings via UI (#2004)

Co-authored-by: bashrusakh <bashrusakh@users.noreply.github.com>

packages/vscode/src/opencodeConfig.ts

@@ -1666,6 +1666,9 @@ export const updateAgent = (agentName: string, updates: Record<string, unknown>,
   const creatingNewMd = isBuiltinOverride;
 
   for (const [field, value] of Object.entries(updates || {})) {
+    // Skip undefined values — they would overwrite existing frontmatter fields with nothing
+    if (value === undefined) continue;
+
     if (field === 'prompt') {
       if (value === null) {
         if (mdExists || creatingNewMd) {