feat: add activity mapper guidance

domains/ui/skills/activity-mappers/skill.md

@@ -0,0 +1,28 @@
+---
+name: activity-mappers
+description: >-
+  Guidance for Activity List and Activity Item changes: start in
+  `@metamask/client-utils/mappers` to share behavior across the mobile and
+  extension apps. Prefer API data, or the keyring transaction for non-EVM;
+  avoid massaging local state. Use when changing Activity List, Activity Item,
+  or their data mapping.
+---
+
+# Activity Mappers
+
+## When To Use
+
+- Changing the Activity List or Activity Item
+- Changing the data used to render an Activity List or Activity Item
+
+## In Scope
+
+- Extension: `ui/pages/activity`, `ui/pages/details`, `shared/lib/activity`
+- Mobile: `app/components/views/Activity`, `app/components/views/ActivityDetails`, `app/util/activity-adapters`
+
+## Workflow
+
+1. Start in [`@metamask/client-utils/mappers`](https://github.com/MetaMask/core/tree/main/packages/client-utils/src/mappers).
+2. Prefer data from the API. For non-EVM activity, use the keyring transaction.
+3. Avoid massaging local state because it does not exist in other instances.
+4. Add or update the unit tests in the mappers package.