Function write_daemon_keypair 

pub async fn write_daemon_keypair(
    daemon_name: &str,
    keypair: &Keypair,
) -> Result<()>

Write a daemon’s keypair to disk. Private key gets 0600, public key gets 0644.

Uses write-to-temp-then-rename for atomicity, same pattern as write_bus_keypair().

Errors

Returns an error if writing the keypair files fails.