docs: state the contributor-license policy (issues yes, code no)

CONTRIBUTING.md

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+<!-- SPDX-License-Identifier: AGPL-3.0-or-later -->
+
+# Contributing to LooperCat
+
+**Short version: issues yes, code no.**
+
+## Bug reports, questions and ideas — welcome
+
+Open an issue. A bug you hit, a question about how something behaves, a feature you miss — all
+of these are genuinely useful and get acted on. If a fix or a feature lands because you raised
+it, you are credited in the release notes.
+
+## Code — not accepted
+
+Pull requests containing code are closed unmerged, regardless of how good the change is. This is
+a standing policy, not a verdict on your patch.
+
+**Why.** LooperCat is AGPL-3.0-or-later and the copyright is held solely by its authors, Oleh
+Tsymaienko and Alisa Lafoks (Darwin's Cat). A merged contribution leaves its author holding
+copyright on those lines permanently. From that moment the project can no longer change its own
+licensing terms — dual-licensing, granting an exception, shipping it inside something else —
+without tracking down every past contributor and obtaining each one's agreement. Nobody can
+foresee a decade of a project's life; declining code costs nothing today, so that is the side we
+err on. The same policy governs
+[felitronics-core](https://github.com/darwinscat/felitronics-core/blob/main/CONTRIBUTING.md),
+which this project builds on.
+
+If your patch would have been genuinely good, **describe the change in an issue instead**. Ideas
+are not copyrightable — only their expression is — so a well-described problem or approach can be
+implemented here freely. You get the fix you wanted, with credit, and the project keeps its
+options open.
+
+Describe it in prose, though — **please do not paste code into the issue**. A pasted patch brings
+the same copyright question in through the side door, and code that has been read cannot be
+unread: someone who has seen your implementation can no longer write that part independently. The
+problem, the approach and the behaviour you expect are what is useful here, and none of that needs
+a diff.
+
+## What this policy does NOT do
+
+It takes away none of the freedoms the AGPL grants you. You may use LooperCat, study it, modify
+it, and distribute your own modified version under the same licence. Forking is expressly fine.
+This is only about what gets merged *here*.
+
+## Third-party code
+
+Third-party dependencies keep their own licences and are unaffected by the policy above. New ones
+must be AGPL-compatible (BSD / MIT / Apache-2.0 / MPL-2.0).
+
+The LooperCat and Darwin's Cat names and logos are trademarks and are *not* covered by the code
+licence.