I'd like to use folder actions to attach the name of the folder images are dropped into in the Description field as Photoshop sees it.
Have a look at ExifTool. https://www.sno.phy.queensu.ca/~phil/exiftool/
Somthing like exiftool *.jpg -description=%%d should do the trick. (Just guessing, never tried the same thing myself)
However, ExifFool does support so called pseudo tags for file and directory names for exactly this purpose (and the other way around, setting the filename based on other Metadata).