chore(repo): Add theming architecture reference (#8596)

Author: alexcarpenter

.changeset/empty-oigyvq.md

@@ -0,0 +1,2 @@
+---
+---

AGENTS.md

@@ -6,3 +6,7 @@ Clerk's JavaScript SDK and library monorepo.
 
 - Non-major releases in `packages/clerk-js` and `packages/ui` are pushed out to consuming applications without requiring explicit package updates. Extra care must be put into any changes to these packages.
 - The API exposed from the core Clerk class in `packages/clerk-js/src/core/clerk.ts` is a contract that is depended on by internal and external consumers. Changes to this API must be done in a major version to avoid breakage.
+
+## References
+
+- For questions about theming, appearance customization, or the styled system, see `references/theming-architecture.md`.