Use one duration parser and fix the --older-than months example (closes #123)

Two functions named parseDuration existed with different grammars. --older-than and --keep-newer-than both already parsed through the one in internal/vaultik; the richer copy in internal/cli/duration.go was reachable only from its own test. Kept the live-path parser and deleted the unused one, so no flag's accepted grammar changes. README documented 6m as the months example for --older-than, but m is minutes, so that command deleted every snapshot older than six minutes; corrected it to 6mo and put both flags' help on one example list stating m is minutes and mo is months. The parser now rejects negatives it used to accept (-5h) or silently made positive (-5d).

Model: opus-4-8

internal/cli/snapshot.go

@@ -141,7 +141,8 @@ specifying a path using --config or by setting VAULTIK_CONFIG to a path.`,
 			"orphaned blobs")
 	cmd.Flags().StringVar(&opts.KeepNewerThan, "keep-newer-than", "",
 		"With --prune: keep snapshots newer than this duration "+
-			"(e.g. 4w, 30d, 6mo) instead of only the latest")
+			"(e.g. 30d, 4w, 6mo, 1y; m is minutes, mo is months) "+
+			"instead of only the latest")
 
 	return cmd
 }
@@ -204,7 +205,8 @@ restrict the operation to specific snapshot names.`,
 	cmd.Flags().BoolVar(&opts.KeepLatest, "keep-latest", false,
 		"Keep only the latest snapshot of each name")
 	cmd.Flags().StringVar(&opts.OlderThan, "older-than", "",
-		"Remove snapshots older than duration (e.g., 30d, 6m, 1y)")
+		"Remove snapshots older than duration "+
+			"(e.g. 30d, 4w, 6mo, 1y; m is minutes, mo is months)")
 	cmd.Flags().BoolVar(&opts.Force, "force", false, "Skip confirmation prompt")
 	cmd.Flags().StringArrayVar(&opts.Names, "snapshot", nil,
 		"Restrict to snapshots with these names (repeat for multiple)")