docs(pi): import Callout explicitly, as the sibling block docs do

`fumadocs-ui/mdx`'s `defaultMdxComponents` already provides `Callout`, so the
callout added earlier rendered fine without this — but logs.mdx, credential.mdx,
and response.mdx all import it explicitly and pi.mdx was the outlier. Not a
build fix: the docs Vercel deployment is failing on staging HEAD as well.

Author: icecrasher321

apps/docs/content/docs/en/workflows/blocks/pi.mdx

@@ -4,6 +4,7 @@ description: The Pi Coding Agent block runs an autonomous coding agent on a real
 pageType: reference
 ---
 
+import { Callout } from 'fumadocs-ui/components/callout'
 import { BlockPreview } from '@/components/workflow-preview'
 import { FAQ } from '@/components/ui/faq'