add asset licensing notices

Adds ASSETS.md, NOTICE.md, per-directory READMEs, third-party license
texts, a CI check that notices don't rot, and README forking-guideline
updates. Carves commissioned artwork, the licensed icon system, and
Bluesky marks out of the blanket MIT license. See LEGMS-208.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

assets/app-icons/README.md

@@ -0,0 +1,16 @@
+The app icons in this directory are trademarks of Bluesky Social
+PBC — the butterfly mark and its variants, as shipped to the iOS App Store and Google Play.
+
+**These are not covered by the [MIT license](../../LICENSE) that applies to the rest of this
+repository.** Use of Bluesky's marks is governed by our Trademark Policy and Brand Guidelines,
+not by the license on our source code.
+
+You may say that your app is a client for Bluesky, or that it is based on the Bluesky app. You
+may not use our marks as the identity of your own product, or in any way likely to suggest that
+Bluesky publishes, endorses, or supports it.
+
+If you are forking this repository, replace these files with your own icons. Shipping an app that
+looks like Bluesky is also a problem under the app stores' own rules on copycat apps, quite apart
+from trademark.
+
+See [`ASSETS.md`](../../ASSETS.md) for the full asset licensing picture.