chore(reuse): add SPDX header to env example

Signed-off-by: Vitor Mattos <vitor@php.rio>

Author: vitormattos

.env.example

@@ -1,3 +1,6 @@
+# SPDX-FileCopyrightText: 2026 LibreCode coop and contributors
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
 # Local sandbox setup for nfse-php smoke tests
 # Copy to .env.local and fill the secret values.