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_flutter/README.md

@@ -13,3 +13,18 @@ A great starting point for learning Serverpod is our documentation site at:
 To run the project, first make sure that the server is running, then do:
 
     flutter run
+
+## Licence
+
+Copyright (C) 2026 The Halkyon Learn contributors
+
+This program is free software: you can redistribute it and/or modify it under the
+terms of the GNU Affero General Public License as published by the Free Software
+Foundation, either version 3 of the License, or (at your option) any later version.
+
+This program is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+PARTICULAR PURPOSE. See the GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public License along with
+this program (`LICENSE`). If not, see <https://www.gnu.org/licenses/>.