fix(client): align the icon button's ring with the shared ring colour

Master's newest agent-preset sheet carries both the new `.helpButton` and the
retained `.iconButton`, whose focus ring still declared the brand token
directly. The stylesheet corpus spec rejects that form, so it now reads
`--dsw-focus-ring-color` with the brand-token fallback.

packages/client/ui-agent-preset/src/client/AgentPresetSection.module.css

@@ -338,7 +338,7 @@
 }
 
 .iconButton:focus-visible {
-  outline: 2px solid var(--dsw-alias-brand-primary);
+  outline: var(--dsw-focus-ring-width) solid var(--dsw-focus-ring-color, var(--dsw-alias-brand-primary));
   outline-offset: -1px;
 }