clarify which variant of Ed25519 OpenPGP uses
RFC 8032 defines three variants: Ed25519
("pure"), Ed25519ctx
("context"), and Ed25519ph
("prehash").
I think OpenPGP is doing the "pure" variant, but it's doing it over the hash of the message, rather than the message itself. This is similar to the "prehash" variant, but not quite.
It'd be good to include some text acknowledging this slight divergence from RFC 8032.