So, I pulled the example project from github, and it seems to be missing the Handelabra.Sentinels library where all the necessary engine classes like HeroCharacterCardController seem to be. I see in the references that there's one called SentinelsEngine and EngineCommon, but both seem to be missing for me.
I looked through the "how to get started" page and don't see anything there as to where to get this library (I probably missed it, but I've looked a few times now).
And it doesn't seem to be a nuget package I can find anywhere.

Where do we go to get that base library to start writing our own mods? Obviously smarter people than I have found it since mods are already on the workshop, so this isn't a criticism of you, this is just my own incompetence showing.

Never mind, I am incompetent. For anyone else struggling with this, its because I was looking for Handelabra.Sentinels in the setup page, not for the SentinelsEngine library.
It says in the instructions that those are found in your steam library folder.
C:\Program Files\Steam\steamapps\common\Sentinels of the Multiverse\Sentinels_Data\Managed

The documentation explains how to do that.

Like is

LMAO