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/images/README.md

@@ -0,0 +1,17 @@
+The product imagery in this directory — onboarding illustrations,
+chat backgrounds, feature announcement graphics, and similar — is **not covered by the
+[MIT license](../../LICENSE)** that applies to the rest of this repository.
+
+Some of it is commissioned artwork that we are not able to sublicense. Rather than have you work
+out file by file which is which, treat the whole directory as not licensed for your use.
+
+If you are forking this repository, replace these files or ship without them.
+
+`germ_logo.webp` is the mark of [Germ Network](https://germnetwork.com), a third party. It is
+theirs, not ours, and we can neither grant nor withhold permission to use it.
+
+**Internal note:** this directory is carved out of the MIT license as a whole. If you are adding
+an asset that we do want forks to be able to reuse, put it somewhere else rather than adding an
+exception here — the carve-out only stays maintainable if the directory boundary means something.
+
+See [`ASSETS.md`](../../ASSETS.md) for the full asset licensing picture.