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For AI vs AI select VS Computer, choose a valid AI deck as your deck, and hold the A key on the keyboard while pressing the load button.  Although all this will do is remind me how bad my AI is at managing its climaxes, choosing standby targets, and how it keeps playing a 5th character at times when it's supposed to keep an open backstage slot.  The AI has the ability to log messages to the chat which helps with debugging, however there is no way to enable this in the current version as the print line is commented out, I'll see if in the next version I can make it print while in debug mode (ctrl+d).
To add an AI you will need to add AIChoice lines to the card data file for the cards you want to it to use, otherwise it won't know the significance of any of the decisions it needs to make while selecting effect targets.  It's also worth noting that the AI script itself does not have all the commands fully programmed, only the ones it needs to play ReZero Emilia and Mushoku Tensei.  (Persona 5 and Axis Church use an old iteration of the AI and have completely different code)

One thing I will not do is make AI for a deck that has a chance to get hit by a ban list, so the entire current meta is out.

This game is made in unity so there is no command line.  The AI simply automates the inputs that would normally be made by a user.  I personally judge the AI's quality by how well it performs against a human player.