agent v0.51.0: local vzdump retention default (--prune-backups keep-last=3)

The preventive counterpart to host_disk + storage_fill detectors: the periodic local
whole-guest vzdump now prunes its own old archives (keep-last=3, clamped >=1) so a box
can't refill its own root via its own backups. Local target only — PBS never pruned
(resolved via ListStorage; fail-safe skip on unknown). Seeded in host-install.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HxLA1mZurFq9kt8hneFeCs

internal/backup/runner_snapshot_test.go

@@ -22,7 +22,7 @@ func TestBackupWithSnapshotHook_FiresOnMarker(t *testing.T) {
 		logLines:   []string{"INFO: backup mode: snapshot", "INFO: create storage snapshot 'vzdump'"},
 		content:    []proxmox.StorageContent{{VolID: "local:backup/vzdump-lxc-9001-x", Content: "backup", VMID: 9001, Size: 100, CTime: 1}},
 	}
-	r := NewBackupRunner(api, "local", "", "", quiet())
+	r := NewBackupRunner(api, "local", "", "", "", quiet())
 
 	var fired int32
 	done := make(chan struct{})
@@ -55,7 +55,7 @@ func TestBackupWithSnapshotHook_StopMode_NeverFires(t *testing.T) {
 		logLines:   []string{"INFO: backup mode: stop"}, // downgraded; no snapshot marker
 		content:    []proxmox.StorageContent{{VolID: "local:backup/vzdump-lxc-9001-x", Content: "backup", VMID: 9001, Size: 100, CTime: 1}},
 	}
-	r := NewBackupRunner(api, "local", "", "", quiet())
+	r := NewBackupRunner(api, "local", "", "", "", quiet())
 
 	var fired int32
 	done := make(chan struct{})