I've been putting pistachio shells in my worm/compost bin with the
rest of the kitchen waste. They don't break down quickly, but (unlike
avocado stones, for example), they aren't big enough to be a nuisance
in the output.

But I've used cocoa shell mulch in the past, and I'm wondering whether
pistachio shells might also be useful as mulch. Any ideas?

Also, is the salt likely to be harmful?