CLI 0.4.0: upload, replace and versions

Publish files from the terminal and swap them behind the same link, with resumable uploads and JSON output for scripts.

New commands: upload, replace, versions, restore, info, login and logout.

Uploads are hashed, sent four parts at a time, verified by Markdrop, and resume when you run the same command again.
--json prints one JSON object and nothing else, for scripts and CI.
markdrop login saves an API token so you don't need MARKDROP_TOKEN in every shell.
markdrop mcp gains upload_file, replace_file, list_file_versions, restore_file_version and get_file_info, so an AI app on your computer can publish files from disk.