minor: note on mutation error handling in CLAUDE.md (#3168)

Author: david-crespo

AGENTS.md

@@ -46,6 +46,7 @@
 # Mutations & UI flow
 
 - Wrap writes in `useApiMutation`, use `confirmAction` to guard destructive intent, and surface results with `addToast`.
+- Mutation error display depends on context. In forms, errors display inline via `submitError={mutation.error}` — do not add `onError` with a toast to the `useApiMutation` call. In `confirmAction`/`confirmDelete` flows, the confirm modal catches the error and shows a toast using `errorTitle` — do not also add `onError` on the mutation, or the user will see two toasts. For standalone actions (fire-and-forget `mutate` calls not wrapped in a confirm modal or form), use `onError` on the mutation to show an error toast.
 - Keep page scaffolding consistent: `PageHeader`, `PageTitle`, `DocsPopover`, `RefreshButton`, `PropertiesTable`, and `CardBlock` provide the expected layout for new system pages.
 - When a page should be discoverable from the command palette, extend `useQuickActions` with the new entry so it appears in the quick actions menu (see `app/pages/ProjectsPage.tsx:100-115`).
 - Gate per-resource actions with capability helpers: `instanceCan.start(instance)`, `diskCan.delete(disk)`, etc. (`app/api/util.ts:91-207`)—these return booleans and have `.states` properties listing valid states. Always use these instead of inline state checks; they centralize business logic and link to Omicron source explaining restrictions.