I prefer not to invent new variables when they aren't strictly
necessary. org-directory is one such variable (although the other path
variables are still necessary).
lang/org now supports the attach:* link abbreviation. Running
+org/attach-sync will index all such links across org files in +org-dir
and delete any files in org-attach-directory that aren't referenced.
TODO: have a separate directory for attach:* attachments and vanilla
attachments?