SSH rejecting key file with extended attributes
I assume your working directory is ~/.ssh/ when you run ssh-add, ls and xattr.

I suspect there is a subdirectory named dev (i.e., ~/.ssh/dev/). Check for it with ls -lFd dev.

ls normally lists the contents of a directory. Adding the -d option lists a directory as a directory itself.

Further, I suspect that there is a file, ~/.ssh/dev/dev which is what ls -lF is displaying the information about.

If my suspicion is correct, xattr dev displays the extended attributes of the subdirectory named dev (not the file dev/dev). The directory attributes probably are not the cause of this problem.

If this is confusing, ls -lFR might help you see what's happening (the -R option recursively lists subdirectories).

Finally, if my suspicion is correct and you're really trying to add a private key identity file named ~/.ssh/dev/dev to the ssh authentication agent, try ssh-add dev/dev (or perhaps better, ssh-add ~/.ssh/dev/dev, which should work no matter the permissions of the enclosing directory.