agent: the restore test takes only archives of a guest that still exists (R-689, second half)

Measured on demo-hp right after v0.135.0: with the golden skipped the pick fell to a leftover
archive of guest 9100, deleted in August. "does not exist" skips it; any other lookup error
makes the tier unknown. Red-proofed.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

internal/backup/runner.go

@@ -359,6 +359,7 @@ func (r *BackupRunner) PickSettledRestoreCandidateOn(ctx context.Context, target
 	}
 	var best string
 	var bestCTime int64 = -1
+	known := map[int]bool{} // vmid → the guest exists on this node (asked once per vmid per pick)
 	for _, e := range contents {
 		if e.Content != "backup" {
 			continue
@@ -371,6 +372,25 @@ func (r *BackupRunner) PickSettledRestoreCandidateOn(ctx context.Context, target
 			r.noteNotAGuestBackupOnce(e, why)
 			continue
 		}
+		// R-689 (v0.136.0): … OF A GUEST THAT STILL EXISTS here. Measured on demo-hp 2026-09-27 right after
+		// v0.135.0: with the golden skipped, the pick fell to `vzdump-lxc-9100-2026_08_21…`, a leftover of a
+		// guest deleted in August — proving nothing about any guest this box runs. "Does not exist" skips the
+		// archive; any OTHER lookup failure is returned, so the tier reads UNKNOWN, never "nothing to prove".
+		if _, seen := known[e.VMID]; !seen {
+			_, err := r.api.GuestConfig(ctx, e.VMID)
+			switch {
+			case err == nil:
+				known[e.VMID] = true
+			case strings.Contains(err.Error(), "does not exist"):
+				known[e.VMID] = false
+			default:
+				return "", time.Time{}, fmt.Errorf("checking whether guest %d still exists: %w", e.VMID, err)
+			}
+		}
+		if !known[e.VMID] {
+			r.noteNotAGuestBackupOnce(e, fmt.Sprintf("guest %d no longer exists on this node", e.VMID))
+			continue
+		}
 		if !notAfter.IsZero() && e.CTime > notAfter.Unix() {
 			continue // not settled yet — a newer archive is not a reason to re-prove an older one
 		}