Lift duplicated sops/age and confirm-prompt logic into scripts/lib/

scripts/backup-admin-key.sh, rotate-admin-key.sh, and sync-host-keys.sh
each independently resolved sops/age's default key-file path, derived an
age pubkey from an identity file, and (two of them) ran `sops updatekeys`
the same way -- now shared via scripts/lib/sops-age.sh. Also extracted the
"type X to confirm" prompt duplicated across create-proxmox-resource.sh
and sync-host-keys.sh into scripts/lib/confirm.sh. Pure extraction, no
behavior change -- each call site produces identical commands/output to
before.

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

scripts/create-proxmox-resource.sh

@@ -46,6 +46,8 @@ repo_root="$(cd "$(dirname "$0")/.." && pwd)"
 source "${repo_root}/scripts/env.sh"
 # shellcheck source=lib/nix-eval.sh
 source "${repo_root}/scripts/lib/nix-eval.sh"
+# shellcheck source=lib/confirm.sh
+source "${repo_root}/scripts/lib/confirm.sh"
 
 sync_keys="${repo_root}/scripts/sync-host-keys.sh"
 
@@ -230,8 +232,7 @@ cmd_modify() {
   fi
 
   echo
-  read -rp "Type the VMID (${vmid}) to confirm these changes: " confirm
-  if [[ "$confirm" != "$vmid" ]]; then
+  if ! confirm_typed "$vmid" "Type the VMID (${vmid}) to confirm these changes: "; then
     echo "Cancelled -- input didn't match ${vmid}."
     exit 1
   fi
@@ -429,8 +430,7 @@ REMOTE_SCRIPT
       echo "  - ${kind} VMID ${id} (${n})"
     done
     echo
-    read -rp "Type the hostname (${host}) to confirm destroying the above and replacing it: " confirm
-    if [[ "$confirm" != "$host" ]]; then
+    if ! confirm_typed "$host" "Type the hostname (${host}) to confirm destroying the above and replacing it: "; then
       echo "Cancelled -- input didn't match ${host}." >&2
       exit 1
     fi