Do not prune after a backup that skipped unreadable files (closes #382)

snapshot create --skip-errors --prune pruned even when files were
skipped, so by default it deleted the previous snapshot of that name,
the only one still holding a file that had become unreadable, along
with blobs only that snapshot referenced.

The prune is now skipped exactly when the run exits non-zero for
entries --skip-errors could not read or list: older snapshots are
kept, a warning says why, and the run still exits non-zero. A file
removed during the run does not count, so such a run still prunes.
The README's --prune text and the flag's help say so.

Model: opus-5-5

internal/cli/snapshot.go

@@ -132,7 +132,8 @@ on Linux), then /etc/vaultik/config.yml.`,
 		"Run in cron mode (silent unless warning or error)")
 	cmd.Flags().BoolVar(&opts.Prune, "prune", false,
 		"After backup, drop older snapshots of the same name and remove "+
-			"orphaned blobs")
+			"orphaned blobs (not after a backup that skipped unreadable "+
+			"files)")
 	cmd.Flags().StringVar(&opts.KeepNewerThan, "keep-newer-than", "",
 		"With --prune: keep snapshots newer than this duration "+
 			"(e.g. 30d, 4w, 6mo, 1y; m is minutes, mo is months) "+