@stephenw10
Thanks for the pointer on passing the -y flag. Prior to seeing this, I was able to install the sudo package and run the command in an SSH shell. After the "missing" packages were installed, things seem to be correct now.
It is troubling that there was not an explicit notification from pfSense warning that the original upgrade had failed. This seems like a significant omission to me.
When using the CLI available from the GUI (System / Command Prompt), I could not figure out a way for it to accept interactive input. I see that the documentation seems to be consistent with this behavior: "Interactive commands, such as vi will fail similarly, or may exit due to other issues with the terminal being non-interactive."
Thanks again for your help!