The keybase: URI is semantically incorrect

Not a big deal, but the Keybase URI shouldn't start with `keybase://`.

For example, in the URI `keybase://private/user`, `private` is the host and `/user` is the path. It wouldn't make sense to add a port number, for example: `keybase://private:1234/user`; or change the host, for example: `keybase://google.com/user`.

A better fit would be for the URI to start with `keybase:/` For example, in the URI `keybase:/private/user` the path is `/private/user`.

Sorry to bring this to your attention if there is a specific reason for this choice.

The issue does not name files, tests, or an entry point. First locate the Keybase URI parsing and construction code, then trace its callers to identify the affected representations. Done means the URI uses the proposed keybase:/private/user form consistently without breaking existing consumers.

Written by the indexing model from the issue text.