v0.6.0-rc1: slice 6 Phase A — backup + the self-restore-test (local target)

The guest-level backup layer + the journaled self-restore-test (restore→boot→verify→
teardown) that closes "a backup you haven't restored isn't a backup". All benign
(reuses the slice-4 classifier/gate/journal; no new destructive class/crypto). Local
target only; PBS = Phase B. Restore to a NEW guest only. Backups crash-consistent.

- proxmox: DestroyLXC, VzdumpOptions.Notes (notes-template), LatestBackupVolID.
- reconcile: Engine.RunRestoreTest (journal Scratch entry BEFORE mutation; net link-down
  pre-boot; defer teardown always; benign gated destroy) + Recover extended to reap a
  leaked scratch guest (Scratch flag, special-cased before the UPID path; idempotent).
- internal/backup: runner (vzdump + archive resolve + bulk-gap = backup!=1) + cadence
  scheduler (4th daemon goroutine, default 24h) + in-memory report store.
- hub: Backup/RestoreTest filled; collector seams; cross-repo golden byte-identical +
  bidirectional key-set tests; hub handler logs a FAILED restore-test prominently.
- config BackupConfig (band 990000-990009 default); --selftest=backup / restore-test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

configs/agent.example.json

@@ -36,5 +36,12 @@
     "timeout_seconds": 30,
     "ca_file": ""
   },
+  "backup": {
+    "local_backup_target": "local",
+    "restore_storage": "local-lvm",
+    "restore_test_cadence_seconds": 0,
+    "scratch_vmid_min": 990000,
+    "scratch_vmid_max": 990009
+  },
   "log_level": "info"
 }