README: running quak backup from cron, and its exit codes (closes #170)

A new README section shows how to run quak backup unattended: log in once
as the job's user, a crontab with a backup every night and --verify on
Sundays, output appended to a log file, and the HOME and XDG_DATA_HOME the
job needs to find the saved session. A table gives each exit code as the
code returns it, and says which failures the next run retries. The
introduction now lists everything the backup keeps for each file.

Judgement call: the --verify run takes Sunday's slot rather than a second
job that night, since an overlapping run would exit 2.

Model: opus-5-5

TODO.md

@@ -25,6 +25,15 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-06: The README says how to run `quak backup` from cron (issue 170):
+  log in once as the job's user, a crontab with a backup every night and
+  `--verify` on Sundays appending to a log file, and the `HOME` and
+  `XDG_DATA_HOME` the job needs to find the saved session. A table gives each
+  exit code: 0, the backup is complete; 1, files are in `failures.json` or
+  another error stopped the run; 2, another backup of the directory is running;
+  3, there is no usable session. The introduction lists everything the backup
+  keeps for each file.
+
 - 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,