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/splash/README.md

@@ -0,0 +1,19 @@
+The load-screen illustration in this directory was commissioned by Bluesky Social PBC from
+illustrator [Owen D. Pomery](https://owenpomery.com/work), through his agent
+[Brilliant Artists Ltd](https://brilliantartists.co.uk/). Copyright in the artwork remains with
+the artist.
+
+**This artwork is not covered by the [MIT license](../../LICENSE) that applies to the rest of
+this repository.** Our license is limited to Bluesky's own products and channels, it is exclusive
+to us, and it does not permit us to sublicense the artwork or to distribute modified versions
+of it.
+
+If you are forking this repository, replace these files. Because our license is exclusive, the
+artwork is not available for separate third-party licensing while that license runs — please do
+not approach the artist or his agent for permission.
+
+Adding a file here? It is carved out of the MIT license by directory, so a new file inherits
+that treatment automatically. If the file is not commissioned artwork covered by this notice,
+put it somewhere else.
+
+See [`ASSETS.md`](../../ASSETS.md) for the full asset licensing picture.