chore(license): relicense from MIT to AGPL-3.0-or-later

CONTRIBUTING.md

@@ -1,5 +1,58 @@
 # Contributing
 
+## License and Developer Certificate of Origin
+
+This project is licensed under [AGPL-3.0-or-later](./LICENSE). By submitting a contribution, you agree that your contribution is licensed under the same terms.
+
+### Sign-off requirement
+
+We use the [Developer Certificate of Origin](https://developercertificate.org/) (DCO) v1.1 to certify the provenance of contributions. Every commit must include a `Signed-off-by:` trailer:
+
+```text
+Signed-off-by: Your Real Name <your.email@example.com>
+```
+
+Add it automatically with `git commit -s`. The name and email must match the commit author identity. CI rejects commits without a sign-off.
+
+### Full DCO text
+
+```
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project and the open source license(s) involved.
+```
+
 ## Quality Gate
 
 All pull requests should pass: