Correct --cron flag help to name warnings as unsuppressed (closes #87)

The --cron help string claimed "silent unless error", but cron mode sets
UI.SetQuiet(true), which suppresses only the seven quiet-aware UI methods;
Warningf and Errorf still print. The snapshot terminus deliberately routes
its end-of-run summary through Warningf so cron delivers something on
success, so a user trusting the old text could read normal output as a
failure signal. The string now reads "silent unless warning or error",
matching the README wording corrected in #84. No behavior change.

Model: opus-4-8

internal/cli/snapshot.go

@@ -135,7 +135,7 @@ specifying a path using --config or by setting VAULTIK_CONFIG to a path.`,
 	}
 
 	cmd.Flags().BoolVar(&opts.Cron, "cron", false,
-		"Run in cron mode (silent unless error)")
+		"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")