docs: licences and how to contribute

The app and the client package are under AGPL-3.0-or-later, the server under SSPL-1.0, as decided for before the first publish: the server builds on Serverpod's core, which is SSPL, while the app links only BSD-3 packages and the client is generated from our own models. "Or later" keeps the code compatible with future AGPL versions without asking every contributor. Each package carries the official text in its LICENSE (from gnu.org and the SPDX licence data) and a notice in its README; LICENSE.md explains the split and covers the files outside the packages.

CONTRIBUTING.md: contributions come under the licence of the package they change, with no copyright assignment; every commit carries a Signed-off-by certifying the Developer Certificate of Origin, quoted verbatim; tool/check.sh runs before pushes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_server/README.md

@@ -8,3 +8,10 @@ Start your server by running:
     serverpod start
 
 When you are finished, you can shut down the running server with `Q`.
+
+## Licence
+
+Copyright (C) 2026 The Halkyon Learn contributors
+
+Licensed under the Server Side Public License, version 1 (`SSPL-1.0`); the full text
+is in `LICENSE`.