Don't build a river
I'm not a fan of scribe-fitting to extremely irregular surfaces like this. Even if you do a great job it'll look like a child drew a river down the wall. You simply can't get it to look "professional", and it'll draw the eye to the unfortunate situation more so than now.
Ideally the stone wall would've been fit to the wooden wall, with flat cuts on the ends of each stone to fit well against the wood. That's not an option here, even if we consider cutting a channel into the stone in which to insert paneling--the existing wall is in the way of that work. I've done exactly that for remodeling jobs with good success, though.
Sometimes in art, simpler is better
My approach would be to leave it mostly as-is. Since there's a good lap of the stone wall behind the wooden wall, you don't have an odd void to deal with. I would simply remove the existing ripped end paneling board and replace it with a single board that's as wide as the resulting gap to the edge of the framing, putting a bullnose edge on both sides to match the other paneling. If the existing end board is actually full-width, a new strip of paneling (with its edges shaped to match the other boards) would also work.
This will leave a clean, straight line that doesn't attract undue attention. Paint the wall and be happy.