Cannot connect using tnsnames.ora but can connect by substituting its details
Thank you to Wernfried Domscheit for pointing me to sqlnet.ora.  This file was not referenced in the documentation I was using so I had no idea it existed.  After visiting an Oracle host and finding the file in $ORACLE_HOME/network/admin, I copied it to $TNS_ADMIN dir and minimized it through testing to the one setting (no others were needed) Wernfried referenced:

# this setting = my local domain
NAMES.DEFAULT_DOMAIN = DOMAIN


and my preferred connection method of

$SQLPLUS user@prod


succeeded.  I will speak with the Oracle DBAs about this file and it's proper inclusion in this shared dir.