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/fonts/inter/README.md

@@ -0,0 +1,15 @@
+[Inter](https://rsms.me/inter/) is a typeface by Rasmus
+Andersson, licensed under the SIL Open Font License, Version 1.1.
+
+The full license text is in [`OFL.txt`](./OFL.txt) and must travel with these files.
+
+```
+Copyright (c) 2016 The Inter Project Authors (https://github.com/rsms/inter)
+```
+
+**You may redistribute these files.** Nothing in [`ASSETS.md`](../../../ASSETS.md) restricts them
+— they are here because the OFL requires the license text to accompany the font, not because they
+are carved out of our [MIT license](../../../LICENSE).
+
+**One thing to watch:** OFL 1.1 includes a Reserved Font Name provision. If you modify or subset
+the font, you may not distribute the result under the name "Inter."