Relicense under AGPL-3.0-or-later

The same licence as ihasmail, which this tool deploys. The network clause
is the part that matters for a deploy tool: a modified copy offered as a
hosted service that deploys for other people has to make its source
available to them, which GPL would not require. Running the tool for your
own host is unaffected.

v2026.9.13 was released under GPL-3.0-or-later and stays so; this applies
from the next release.

internal/config/config_test.go

@@ -1,5 +1,5 @@
 // SPDX-FileCopyrightText: 2026 Coffey Labs
-// SPDX-License-Identifier: GPL-3.0-or-later
+// SPDX-License-Identifier: AGPL-3.0-or-later
 
 package config