Add scripts/backup-admin-key.sh to back up the local sops admin key

Companion to rotate-admin-key.sh: copies whatever age identity sops/age
itself would resolve (or an explicit --key-file) to a given destination
path with 0600 perms, validating it's a real identity and round-tripping
the derived public key before/after the write so a corrupted copy is
caught immediately rather than discovered later during a restore.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CLAUDE.md

@@ -117,6 +117,16 @@ Beyond `codex-setup.sh`/`codex-maintenance.sh` above, `scripts/` also has:
   for the manual steps `sync-host-keys.sh`/`create-proxmox-resource.sh`
   print when they bootstrap a brand-new, not-yet-trusted key on a machine
   with no prior admin access.
+- `scripts/backup-admin-key.sh <dest-path> [--key-file <path>] [--force]
+  [--dry-run]` — copies the local sops age key (source resolution matches
+  sops/age itself: `$SOPS_AGE_KEY` inline, then `--key-file`, then
+  `$SOPS_AGE_KEY_FILE`, then the XDG default) to an arbitrary destination
+  path with `0600` permissions, validating it's a real age identity and
+  round-tripping the public key before and after the write. Refuses to
+  overwrite an existing `<dest-path>` without `--force`. Purely a local
+  filesystem copy — never touches `.sops.yaml`/`secrets/*.yaml` or the
+  repo at all. The resulting file is exactly what `rotate-admin-key.sh`
+  expects as its backup-key argument.
 
 `sync-host-keys.sh`, `create-proxmox-resource.sh`, and
 `rotate-admin-key.sh` genuinely mutate real state when run for real (not
@@ -125,6 +135,9 @@ containers, real revocation of decrypt access. They require the
 operator's own SSH/sops access, which an agent session doesn't have — but
 don't suggest running any of them non-dry-run without the operator's
 explicit go-ahead even if it becomes technically reachable.
+`backup-admin-key.sh` only writes a key copy to a path the operator gives
+it — lower-stakes than the others, but it still handles a real private
+key, so treat its destination path choice as the operator's call too.
 
 ## Architecture