How should I add an AFCI to this panel?
You have a panel with 12 spaces and you are using 4. Just move the red wire to an AFCI in a new space in the panel.   

I would also move the lower black wire to where the red wire is now, so it becomes visually obvious that both wires are meant to be on the same pole.   I'm all for using a rainbow of colors in conduit/THWN, but in residential use with NM cable, red always means something special in some way.  

On a tandem (I call them double-stuff) breaker, both circuits are on the same pole.  It is very easy for someone to confuse them for a 2-pole breaker, which MWBC's require for safety.  So when an electrician sees a panel with a double-stuff with red and black wires, he'll pause and follow the wires just to make sure that mistake wasn't made.