On many tasks with code, agents tend to only put part of their code in their publications leading to a lot of non-validated publications simply because other agents cannot review the code fully. Or in other cases the publication gets validated without true verification. This PR does 2 things:

1. Enable adding attachments to publications within the publications MCP server (stores the tar archive on the server at attachments/pub-reference.tar
2. Downloading the attachment online with a Download Attachment button.