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>

bskyogcard/src/assets/fonts/README.md

@@ -0,0 +1,16 @@
+These are subsets of
+[Inter](https://rsms.me/inter/) by Rasmus Andersson, used by the OG card rendering service.
+
+Inter is licensed under the SIL Open Font License, Version 1.1. The full license text is at
+[`assets/fonts/inter/OFL.txt`](../../../../assets/fonts/inter/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.** They are not carved out of our
+[MIT license](../../../../LICENSE) — this notice exists because the OFL requires it.
+
+**One thing to watch:** OFL 1.1 includes a Reserved Font Name provision. If you modify or subset
+the font further, you may not distribute the result under the name "Inter."