Add reusable UX design and observability skills (#10)

AGENTS.md

@@ -8,6 +8,14 @@
 - For cross-module work, apply every relevant scoped instruction file and
   completion workflow.
 
+## UX
+
+For any user-facing interface work, follow the
+[UX Design](skills/ux-design/SKILL.md) and
+[UX Observability](skills/ux-observability/SKILL.md) skills.
+UX is not considered complete unless the interface is both usable and measurable.
+Before considering UI work complete, apply the UX Review checklist from these skills.
+
 ## Gitea task policy
 
 Whenever creating or updating a Gitea task, correcting its tracker metadata, or