quak backup refuses to run while another backup of the same directory runs, exit 2 (closes #169)

lib.backup() takes a lock, backup.lock in its download directory, made
with proper-lockfile, before its refresh, and removes it when it ends. A
second backup of the directory fails at once with an error naming it;
quak backup prints it as one line and exits 2. A lock untouched for 10
seconds, left by a killed run, is taken over.

Deviation: yarn.lock was regenerated by yarn add in the pinned node image.
Judgement call: a run failing at its refresh leaves the directory, empty.
Judgement call: a lock removed mid-run stops that run with an uncaught
error, the library's default.

Model: opus-5-5

TODO.md

@@ -25,6 +25,15 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-06: Two backups of the same directory never run at once (issue 169).
+  `lib.backup()` takes a lock, `backup.lock` in its download directory, made
+  with `proper-lockfile`, before its refresh, and removes it when it ends,
+  whether it succeeds or fails. A second backup of the directory, from another
+  process or the same one, fails at once with an error naming the directory;
+  `quak backup` prints it as one line and exits 2. A lock that has gone 10
+  seconds untouched, left by a run that was killed, is taken over by the next
+  run.
+
 - 2026-10-06: `quak backup` writes each original's EXIF, XMP and dimensions into
   the file's JSON as `imageMetadata`, what `backup-metadata --exif` records
   (issue 167): for a live photo from its image, for a video nothing, and `{}`