Deploy to Vercel as a TanStack Start app via Nitro

Vercel served 404s: with Hydrogen gone it treated the app as a static
Vite site, but TanStack Start's build has no index.html; pages come from
its server. Add the hosting adapter TanStack documents for Vercel:

- nitro() in vite.config, for 'vite build' only (dev and the nodesandbox
  runtime never load it); on Vercel it writes .vercel/output with the
  static assets and one Node.js function running Start + Hono
- vercel.json: tanstack-start preset, Yarn 4 via Corepack
- 'yarn start' runs the Node build; canonical URLs use the Vercel domain

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

vercel.json

@@ -0,0 +1,6 @@
+{
+  "$schema": "https://openapi.vercel.sh/vercel.json",
+  "framework": "tanstack-start",
+  "installCommand": "corepack enable && yarn install",
+  "buildCommand": "yarn build"
+}