fix(staging): rotate SSH secret handling and env ownership

deploy/validate-staging-env.sh

@@ -24,6 +24,17 @@ if [ "$MODE" != "600" ]; then
   exit 1
 fi
 
+if OWNER="$(stat -c '%u' "$ENV_FILE" 2>/dev/null)"; then
+  :
+else
+  OWNER="$(stat -f '%u' "$ENV_FILE")"
+fi
+EXPECTED_OWNER_UID="${EXPECTED_STAGING_ENV_OWNER_UID:-$(id -u)}"
+if [[ ! "$EXPECTED_OWNER_UID" =~ ^[0-9]+$ ]] || [ "$OWNER" != "$EXPECTED_OWNER_UID" ]; then
+  echo "staging environment file has an invalid owner" >&2
+  exit 1
+fi
+
 require_selector() {
   local key="$1"
   local expected="$2"