ui: AGENT THINKING joins the small mono-caps register (the aborted edit landed)

The round-4 batch threw on an earlier replace and never reached
reasoning.tsx — the trigger row kept text-sm. Now text-xs font-medium
uppercase tracking-wide, matching YOUR THINKING and the card eyebrows;
chevrons align through the 1ch rail. Plus one missed prettyModel pin
(gpt 5.2, no auto-caps).

components/ai-elements/reasoning.tsx

@@ -152,7 +152,10 @@ export const ReasoningTrigger = memo(
     return (
       <CollapsibleTrigger
         className={cn(
-          "flex w-full items-center gap-2 text-muted-foreground text-sm transition-colors hover:text-foreground",
+          /* The heading register (owner round 4, 2026-10-05): the small
+             mono caps of the card eyebrows — matching YOUR THINKING, whose
+             rail offsets it by exactly the one-character difference. */
+          "flex w-full items-center gap-2 text-muted-foreground text-xs font-medium uppercase tracking-wide transition-colors hover:text-foreground",
           className
         )}
         {...props}