Mainplane harness pointer.

GET /<key>  the record of the harness with that ed25519 public key: {"url", "seq", "sig"}
PUT /<key>  {"url", "seq", "sig"}: sig is ed25519 over "<key> <url> <seq>", seq is unix
            milliseconds and must go up. The record expires 30 days after its last PUT.
Keys and signatures are base64url without padding. Check sig yourself; the URL is a hint.