gosdk/zbox: implement purge cmd for feed and stream

a lot of files will be uploaded on `zbox feed` and `zbox stream`. a `zbox pruge` should be implemented to delete stale files

Look at the existing `zbox feed` and `zbox stream` commands in the codebase to understand how files are uploaded and tracked. The new `zbox purge` command will need to identify stale files and delete them. Start by examining the command structure and file management logic, then implement the purge functionality. Test by running the command on a local setup to verify it deletes the intended files.

Scritto dal modello di indicizzazione a partire dal testo della issue.