🤖 refactor: disable Chromatic snapshots for Shimmer animations (#610)

The Shimmer component uses infinite animations (`repeat:
Number.POSITIVE_INFINITY`) which cause visual regression test failures
in Chromatic. Each snapshot shows the animation at a different point,
causing spurious diffs.

**Changes:**
- Disabled Chromatic snapshots for `ReasoningMessage.stories.tsx` (all
stories use Shimmer)
- Disabled Chromatic snapshots for `ActiveWorkspaceWithChat` story
(includes streaming reasoning messages with Shimmer)

**Why `disableSnapshot` instead of other approaches:**
- Can't mock animations - Shimmer is an intentional UI feature we want
in Storybook
- Can't use `pauseAnimationAtEnd` - animation loops infinitely
- Stories remain useful for manual testing and Storybook documentation

_Generated with `mux`_

Author: kylecarbs

src/components/ai-elements/shimmer.tsx

@@ -33,6 +33,7 @@ const ShimmerComponent = ({
         "[--bg:linear-gradient(90deg,#0000_calc(50%-var(--spread)),var(--color-background),#0000_calc(50%+var(--spread)))] [background-repeat:no-repeat,padding-box]",
         className
       )}
+      data-chromatic="ignore"
       initial={{ backgroundPosition: "100% center" }}
       style={
         {