Update license from PMPL-1.0 to AGPL-3.0

Author: hyperpolymath

README.adoc

@@ -174,6 +174,6 @@ The configuration file (`user-config.ncl`) is **gitignored** to ensure your serv
 
 == License
 
-SPDX-License-Identifier: PMPL-1.0-or-later
+SPDX-License-Identifier: AGPL-3.0-or-later
 
 Copyright (c) 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>