README: guard the daily cron scan against an unmounted disk (closes #175)

A disk that is not mounted leaves its mount point an empty directory.
scan then deletes every record under it, and the next scan reads every
file again. The README now says so. Its cron line runs scan only when
mountpoint finds the disks mounted, and one sentence says which mount
points to test.

The Description drops its sentence on copies deleted after the scan:
"Reading the reports" and "Goals" already say it.

Model: opus-5-5

TODO.md

@@ -29,6 +29,9 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- `README.md` says what a disk that is not mounted does to the database. The
+  daily `cron` line runs `scan` only when `mountpoint` reports `/srv` as mounted
+  (2026-11-11, https://git.eeqj.de/sneak/sfdupes/issues/175)
 - `report` and `trees` leave out a copy that was deleted after the scan. Each
   checks only the copy itself with `lstat`: the file, or the directory at the
   top of a tree. A group left with one copy is not reported. A missing `scan`