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

Two parseDuration functions existed with different grammars; only the one in internal/vaultik/helpers.go was reachable from a flag. The unused copy in internal/cli/duration.go is deleted, so no flag accepts anything it did not before.

The README gave 6m as the six-months example for snapshot purge --older-than, but m is minutes: that command removed every snapshot older than six minutes. The example is now 6mo, and the help for --older-than and --keep-newer-than states that m is minutes and mo is months.

The parser now rejects negative durations, which it used to accept or silently make positive.

model: claude-opus-4-8 (implementation, review); claude-fable-5-1 (merge)

Co-authored-by: clawbot <clawbot@noreply.example.org>

internal/vaultik/helpers.go

@@ -33,8 +33,9 @@ func ubytes(n int64) string {
 var (
 	errMalformedSnapshotID = errors.New(
 		"invalid snapshot ID format: expected hostname_snapshotname_timestamp")
-	errInvalidDuration = errors.New("invalid duration")
-	errUnknownTimeUnit = errors.New("unknown time unit")
+	errInvalidDuration  = errors.New("invalid duration")
+	errUnknownTimeUnit  = errors.New("unknown time unit")
+	errNegativeDuration = errors.New("negative durations are not supported")
 )
 
 // Time-unit lengths used by parseDuration.
@@ -138,8 +139,13 @@ func parseSnapshotName(snapshotID string) string {
 
 // parseDuration parses a duration string with support for human-friendly units:
 // d/day/days, w/week/weeks, mo/month/months, y/year/years, plus standard Go
-// duration units (h, m, s).
+// duration units. Following Go, m is minutes and mo is months. A bare number,
+// an unknown unit, and a negative value are all rejected.
 func parseDuration(s string) (time.Duration, error) {
+	if strings.HasPrefix(strings.TrimSpace(s), "-") {
+		return 0, errNegativeDuration
+	}
+
 	d, err := time.ParseDuration(s)
 	if err == nil {
 		return d, nil