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/icons/community/README.md

@@ -0,0 +1,13 @@
+The icons in this directory are the marks of third-party
+services that appear in the Bluesky app — [Leaflet](https://leaflet.pub),
+[Offprint](https://offprint.net), pckt, and standard-site.
+
+**These marks belong to their respective owners.** They are not Bluesky trademarks, they are not
+covered by the [MIT license](../../../LICENSE) that applies to the rest of this repository, and we
+are not in a position to grant or withhold permission to use them. We include them to identify
+those services in our UI.
+
+If you fork this repository and keep these icons, your use of them rests on your own
+nominative-use basis or on permission from the mark owner.
+
+See [`ASSETS.md`](../../../ASSETS.md) for the full asset licensing picture.