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/README.md

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+The user-interface glyphs in this directory are from the
+[Central icon system](https://iconists.co/central) by Iconists (David & Storm GbR). Bluesky
+Social PBC licenses them for use in our own products. **That license is for our own use, and it
+does not extend to you.**
+
+**These icons are not covered by the [MIT license](../../LICENSE) that applies to the rest of
+this repository.** If you want to use them, buy a license from
+[iconists.co](https://iconists.co). There are also good openly licensed icon sets if you would
+rather not.
+
+This notice covers the SVG files at the top level of this directory. It does not cover:
+
+- `flags/` — country-flag-icons, MIT licensed, see [`flags/README.md`](./flags/README.md)
+- `community/` — third-party service marks, see [`community/README.md`](./community/README.md)
+- `logomark.svg`, `newskie.svg`, `verifiedCheck.svg`, `verifierCheck.svg`, `starterPack.svg`,
+  `starter_pack_icon.svg` — Bluesky trademarks and brand assets
+- `custom_logo_japan.svg` — artwork by a Bluesky Japan logo contest entrant
+- `apple_logo.svg`, `android_logo.svg` — Apple and Google trademarks
+
+Each of those is described in [`ASSETS.md`](../../ASSETS.md).
+
+Adding an icon here? If it came from Central, this notice already covers it. If it came from
+anywhere else, add it to `ASSETS.md` so the notice does not go stale.

assets/icons/community/README.md

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+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.