Reject a retention duration longer than about 292 years (closes #289)

parseDuration multiplied each number by the length of its unit with no
limit, so from 293y (or 15251w, 3559mo, 106752d, or a sum such as
200y200y) the result wrapped to a negative duration. The purge cutoff
then lay in the future and every snapshot of the name was selected, for
--older-than and --keep-newer-than alike. Each part is now checked
before it is added, and a total past the largest time.Duration is
rejected as an invalid duration, so nothing is deleted. Go units were
already safe: time.ParseDuration rejects an overflow.

With --prune the backup still runs before the duration is read; only
the prune step fails.

Model: opus-5-5

TODO.md

@@ -22,6 +22,16 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-12: Rejected a retention duration longer than about 292 years
+  ([issue #289](https://git.eeqj.de/sneak/vaultik/issues/289)). The
+  number given to `--older-than` or `--keep-newer-than` was multiplied
+  by the length of its unit with no limit, so from `293y`, `15251w`,
+  `3559mo` or `106752d`, or a sum of parts such as `200y200y`, the
+  result wrapped to a negative duration. The cutoff then lay in the
+  future, every snapshot of the name was deleted, the one just made
+  included, and the command exited 0. Such a duration is now rejected
+  as invalid and nothing is deleted.
+
 - 2026-10-08: Made a backup cancelled under `--skip-errors` stop at once
   ([issue #286](https://git.eeqj.de/sneak/vaultik/issues/286)). After
   Ctrl-C or SIGTERM, phase 2 treated the cancellation error from each