feat(ST-1114): add shared Markdown wrapper, apply to task-detail and story-dialog

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/markdown.tsx

@@ -0,0 +1,21 @@
+import ReactMarkdown from 'react-markdown'
+import remarkGfm from 'remark-gfm'
+import { cn } from '@/lib/utils'
+
+interface MarkdownProps {
+  children: string
+  className?: string
+}
+
+export function Markdown({ children, className }: MarkdownProps) {
+  return (
+    <div className={cn('prose prose-sm dark:prose-invert max-w-none', className)}>
+      <ReactMarkdown
+        remarkPlugins={[remarkGfm]}
+        disallowedElements={['script', 'iframe']}
+      >
+        {children}
+      </ReactMarkdown>
+    </div>
+  )
+}