3.x Rekey S3 Credentials

After rotating IAM credentials in AWS, update the S3 access key and secret stored in your LucidLink 3.x filespace using this script. Run immediately after key rotation to prevent filespace disconnection. The script runs `lucid rekey-s3` against the mounted filespace, so the LucidLink 3.x client must be installed and the filespace connected. A filespace administrator password is required.