Function unlockSubjectWith

Decrypts the subject with the content key agent unlocks: the reference's unlock_subject(id) under the ssh-agent feature. Every unobscured hasSecret object must decode as an EncryptedKey; a key locked through an agent is opened with the identity id names, else the identity stored in the key, else the agent's first Ed25519 identity; a key locked by a password is tried with id as the password. The first key that opens decrypts the subject; one that does not is skipped, whatever the reason.