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/rotate-admin-key.sh

@@ -24,6 +24,8 @@ sops_yaml="${repo_root}/.sops.yaml"
 
 # shellcheck source=env.sh
 source "${repo_root}/scripts/env.sh"
+# shellcheck source=lib/sops-age.sh
+source "${repo_root}/scripts/lib/sops-age.sh"
 
 # sops resolves .sops.yaml by walking up from the process's cwd, not from
 # the target file's own path -- if this script were invoked from somewhere
@@ -53,7 +55,7 @@ EOF
 }
 
 dry_run=0
-new_key_file="${SOPS_AGE_KEY_FILE:-${XDG_CONFIG_HOME:-$HOME/.config}/sops/age/keys.txt}"
+new_key_file="$DEFAULT_SOPS_AGE_KEY_FILE"
 args=()
 
 while [[ $# -gt 0 ]]; do
@@ -93,13 +95,9 @@ backup_key="${args[0]}"
 
 nix_extra_opts
 
-age_pub() {
-  nix-shell "${NIX_OPTS[@]}" -p age --run "age-keygen -y '$1'"
-}
-
 echo "==> Deriving public keys..."
-old_pub="$(age_pub "$backup_key")"
-new_pub="$(age_pub "$new_key_file")"
+old_pub="$(age_pubkey_from_identity_file "$backup_key")"
+new_pub="$(age_pubkey_from_identity_file "$new_key_file")"
 echo "    backup (old admin) key: ${old_pub}"
 echo "    new admin key:          ${new_pub}"
 
@@ -108,12 +106,11 @@ if [[ "$old_pub" == "$new_pub" ]]; then
   exit 1
 fi
 
-current_admin_line="$(grep -E '^  - &admin age1' "$sops_yaml" || true)"
-if [[ -z "$current_admin_line" ]]; then
+current_admin_pub="$(sops_yaml_admin_pubkey "$sops_yaml")"
+if [[ -z "$current_admin_pub" ]]; then
   echo "ERROR: couldn't find a '&admin age1...' line in ${sops_yaml}." >&2
   exit 1
 fi
-current_admin_pub="$(awk '{print $NF}' <<<"$current_admin_line")"
 
 if [[ "$current_admin_pub" != "$old_pub" ]]; then
   echo "ERROR: ${backup_key} doesn't match the current &admin key in .sops.yaml." >&2
@@ -129,9 +126,17 @@ if [[ "${#secrets_files[@]}" -eq 0 ]]; then
   exit 1
 fi
 
+# sops_can_decrypt <key-file> <secrets-file>: used both to confirm the
+# backup key still works before touching anything, and again after
+# rotation to confirm the new key does too.
+sops_can_decrypt() {
+  local key_file="$1" secrets_file="$2"
+  SOPS_AGE_KEY_FILE="$key_file" nix-shell "${NIX_OPTS[@]}" -p sops --run \
+    "sops -d '${secrets_file}'" >/dev/null
+}
+
 echo "==> Confirming the backup key can actually decrypt..."
-if ! SOPS_AGE_KEY_FILE="$backup_key" nix-shell "${NIX_OPTS[@]}" -p sops --run \
-  "sops -d '${secrets_files[0]}'" >/dev/null; then
+if ! sops_can_decrypt "$backup_key" "${secrets_files[0]}"; then
   echo "ERROR: backup key failed to decrypt $(basename "${secrets_files[0]}") -- aborting." >&2
   exit 1
 fi
@@ -162,14 +167,12 @@ echo "    Updated."
 echo "==> Re-encrypting secrets/*.yaml for the new recipient set..."
 for f in "${secrets_files[@]}"; do
   echo "==> $(basename "$f")"
-  SOPS_AGE_KEY_FILE="$backup_key" nix-shell "${NIX_OPTS[@]}" -p sops --run \
-    "sops updatekeys --yes '${f}'"
+  sops_updatekeys "$f" "$backup_key"
 done
 
 echo "==> Verifying the new key can decrypt everything..."
 for f in "${secrets_files[@]}"; do
-  if ! SOPS_AGE_KEY_FILE="$new_key_file" nix-shell "${NIX_OPTS[@]}" -p sops --run \
-    "sops -d '${f}'" >/dev/null; then
+  if ! sops_can_decrypt "$new_key_file" "$f"; then
     echo "ERROR: new key failed to decrypt $(basename "$f") after rotation -- investigate before committing." >&2
     exit 1
   fi