feat(skill): add preprocessing for markdown files before upload

Introduce prepareSkillUploadFile utility that wraps markdown file content
in a JSON payload format before uploading. This ensures consistent handling
of skill files across file upload, folder upload, and drag-and-drop operations.

web/app/components/workflow/skill/utils/skill-upload-utils.ts

@@ -0,0 +1,15 @@
+import { getFileExtension, isMarkdownFile } from './file-utils'
+
+const buildSkillUploadPayload = (content: string) => {
+  return JSON.stringify({ content, metadata: {} })
+}
+
+export async function prepareSkillUploadFile(file: File): Promise<File> {
+  const extension = getFileExtension(file.name)
+  if (!isMarkdownFile(extension))
+    return file
+
+  const content = await file.text()
+  const payload = buildSkillUploadPayload(content)
+  return new File([payload], file.name, { type: file.type || 'text/plain' })
+}