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/backup-admin-key.sh

@@ -20,6 +20,8 @@ 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"
 
 # Pin cwd for the same reason rotate-admin-key.sh does: age/sops calls
 # below should never depend on wherever the caller's shell happened to be.
@@ -43,7 +45,7 @@ EOF
 
 dry_run=0
 force=0
-key_file="${SOPS_AGE_KEY_FILE:-${XDG_CONFIG_HOME:-$HOME/.config}/sops/age/keys.txt}"
+key_file="$DEFAULT_SOPS_AGE_KEY_FILE"
 args=()
 
 while [[ $# -gt 0 ]]; do
@@ -103,13 +105,13 @@ scratch="$(mktemp)"
 trap 'rm -f "$scratch"' EXIT
 ( umask 077; printf '%s\n' "$src_content" > "$scratch" )
 
-src_pub="$(nix-shell "${NIX_OPTS[@]}" -p age --run "age-keygen -y '$scratch'")" || {
+src_pub="$(age_pubkey_from_identity_file "$scratch")" || {
   echo "ERROR: source doesn't look like a valid age identity (age-keygen -y failed)." >&2
   exit 1
 }
 echo "    public key: ${src_pub}"
 
-current_admin_pub="$(grep -E '^  - &admin age1' "$sops_yaml" 2>/dev/null | awk '{print $NF}' || true)"
+current_admin_pub="$(sops_yaml_admin_pubkey "$sops_yaml")"
 if [[ -n "$current_admin_pub" && "$current_admin_pub" != "$src_pub" ]]; then
   echo "NOTE: this key does not match .sops.yaml's current &admin entry (${current_admin_pub})."
   echo "      Backing it up anyway -- this script doesn't require it to be the admin key."
@@ -131,7 +133,7 @@ fi
 mkdir -p "$(dirname "$dest")"
 install -m 600 "$scratch" "$dest"
 
-dest_pub="$(nix-shell "${NIX_OPTS[@]}" -p age --run "age-keygen -y '$dest'")"
+dest_pub="$(age_pubkey_from_identity_file "$dest")"
 if [[ "$dest_pub" != "$src_pub" ]]; then
   echo "ERROR: ${dest} was written but its public key doesn't match the source -- investigate before relying on this backup." >&2
   exit 1