Function signing_key_from_age_bytes 

pub fn signing_key_from_age_bytes(age_key_bytes: &[u8]) -> SigningKey

Derive an Ed25519 signing key from the raw 32-byte age x25519 secret.

The BIP39 mnemonic encodes these same 32 bytes, so the signing key recovers for free from the recovery phrase — no extra words to back up.