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/lib/confirm.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+# Shared "type X to confirm" prompt for scripts/create-proxmox-resource.sh
+# (--modify, and replacing an existing --allow-duplicate-host resource) and
+# scripts/sync-host-keys.sh (--regenerate-all-keys) -- three destructive
+# confirmations that all work the same way (echo the expected value back
+# exactly), kept in one place so the prompt/comparison logic can't drift.
+# Source alongside env.sh:
+#   source "$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)/lib/confirm.sh"
+#
+# Deliberately does NOT print anything on mismatch or decide exit-vs-return
+# -- callers vary on both (a top-level script exits, a subcommand function
+# returns; wording differs too), so that stays at the call site.
+
+# confirm_typed <expected> <prompt>
+# Prints <prompt> via `read -rp`, then reports (via exit status) whether the
+# typed input matched <expected> exactly.
+confirm_typed() {
+  local expected="$1" prompt="$2" input
+  read -rp "$prompt" input
+  [[ "$input" == "$expected" ]]
+}