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/sync-host-keys.sh

@@ -35,6 +35,10 @@ source "${repo_root}/scripts/env.sh"
 source "${repo_root}/scripts/lib/nix-eval.sh"
 # shellcheck source=lib/ssh-host-keys.sh
 source "${repo_root}/scripts/lib/ssh-host-keys.sh"
+# shellcheck source=lib/sops-age.sh
+source "${repo_root}/scripts/lib/sops-age.sh"
+# shellcheck source=lib/confirm.sh
+source "${repo_root}/scripts/lib/confirm.sh"
 
 mkdir -p "$keydir"
 
@@ -78,7 +82,7 @@ ensure_admin_decrypt_key() {
     return
   fi
 
-  local key_file="${SOPS_AGE_KEY_FILE:-${XDG_CONFIG_HOME:-$HOME/.config}/sops/age/keys.txt}"
+  local key_file="$DEFAULT_SOPS_AGE_KEY_FILE"
 
   if [[ -s "$key_file" ]]; then
     echo "Found existing sops age key at ${key_file}."
@@ -97,7 +101,7 @@ ensure_admin_decrypt_key() {
   mkdir -p "$(dirname "$key_file")"
   nix-shell "${NIX_OPTS[@]}" -p age --run "age-keygen -o '${key_file}'" 2>&1 | grep -v "^Public key:" || true
   local new_pub
-  new_pub="$(nix-shell "${NIX_OPTS[@]}" -p age --run "age-keygen -y '${key_file}'")"
+  new_pub="$(age_pubkey_from_identity_file "$key_file")"
 
   cat <<EOF
 
@@ -239,7 +243,7 @@ apply_edit_plan() {
       while IFS= read -r basename; do
         [[ -z "$basename" ]] && continue
         echo "==> secrets/${basename}"
-        nix-shell "${NIX_OPTS[@]}" -p sops --run "sops updatekeys --yes '${repo_root}/secrets/${basename}'"
+        sops_updatekeys "${repo_root}/secrets/${basename}"
       done <<<"$changed"
     fi
   fi
@@ -359,8 +363,7 @@ cmd_regenerate_all() {
   echo "image/tarball before it can decrypt secrets again."
 
   if [[ "$dry_run" -ne 1 ]]; then
-    read -rp "Type REGENERATE to confirm: " confirm
-    if [[ "$confirm" != "REGENERATE" ]]; then
+    if ! confirm_typed "REGENERATE" "Type REGENERATE to confirm: "; then
       echo "Cancelled."
       return
     fi