CLI helper to upload local files to Puter storage

[chindris-mihai-alexandru]

Problem: Users need a simple way to upload PDFs/images to Puter and get a puter_path for proxy/tool use.

Tasks:

• Add puter-auth upload command.
• Return puter_path and size/mime.
• Document usage in README.

Acceptance:

• Uploading a file returns a valid puter_path used by proxy/tooling.