mt-add-image 是做什么的？
Image handler for the Hugo blog newsletter. Adds an image URL to the target newsletter post's Bonus → Images with a detected label. For Substack-hosted images it finds the source post (ByteByteGo and other configured publications) via RSS then sitemap, using the figure caption or post title as the label; for other sites it is best-effort; if no label is found it asks you to type one. Normally invoked by the mt-add-url meta skill after classification, but can be used directly for a known image UR