Add --keep-newer-than flag for rolling retention window

snapshot create --prune now accepts --keep-newer-than <duration> (e.g.
4w, 30d, 6mo) to keep a rolling window of snapshots instead of only
the latest. Supports d/w/mo/y units and combinations (2w3d).

Without --keep-newer-than, --prune still defaults to keep-latest-only.

internal/vaultik/helpers_test.go

@@ -2,6 +2,7 @@ package vaultik
 
 import (
 	"testing"
+	"time"
 )
 
 func TestParseSnapshotName(t *testing.T) {
@@ -37,6 +38,41 @@ func TestParseSnapshotName(t *testing.T) {
 	}
 }
 
+func TestParseDuration(t *testing.T) {
+	tests := []struct {
+		input string
+		want  time.Duration
+		err   bool
+	}{
+		{"30d", 30 * 24 * time.Hour, false},
+		{"4w", 4 * 7 * 24 * time.Hour, false},
+		{"6mo", 6 * 30 * 24 * time.Hour, false},
+		{"1y", 365 * 24 * time.Hour, false},
+		{"2w3d", 2*7*24*time.Hour + 3*24*time.Hour, false},
+		{"1h", time.Hour, false},
+		{"30s", 30 * time.Second, false},
+		{"garbage", 0, true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.input, func(t *testing.T) {
+			got, err := parseDuration(tt.input)
+			if tt.err {
+				if err == nil {
+					t.Fatalf("expected error for %q, got %v", tt.input, got)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error for %q: %v", tt.input, err)
+			}
+			if got != tt.want {
+				t.Errorf("parseDuration(%q) = %v, want %v", tt.input, got, tt.want)
+			}
+		})
+	}
+}
+
 func TestParseSnapshotTimestamp(t *testing.T) {
 	tests := []struct {
 		name       string