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>

README.md

@@ -20,9 +20,19 @@ yarn dev                     # http://localhost:3000
 | Script           | What it does                                              |
 | ---------------- | --------------------------------------------------------- |
 | `yarn dev`       | Vite dev server with SSR and the Hono `/api` routes       |
-| `yarn build`     | Production build to `dist/`                               |
+| `yarn build`     | Production build; Nitro packages the server for the host  |
+| `yarn start`     | Run the Node build from `.output/`                        |
 | `yarn typecheck` | `tsc`                                                     |
 
+## Deploying to Vercel
+
+Push and deploy — no dashboard settings. `vercel.json` selects the TanStack
+Start preset and installs with Yarn 4 via Corepack. During `vite build` the
+Nitro plugin detects Vercel and writes `.vercel/output/` (static assets plus one
+Node.js function running TanStack Start, with Hono serving `/api/*`). Set
+`VITE_SHOPIFY_*` in the project to use a real store; without them it uses
+mock.shop.
+
 ## nodesandbox compatibility
 
 The sandbox runs Vite's native tools as WebAssembly (`NAPI_RS_FORCE_WASI`), and
@@ -37,7 +47,7 @@ its installer resolves from `package.json` ranges rather than `yarn.lock`. So:
   optional dependency itself, and npm on other hosts rejects them
   (`EBADPLATFORM`).
 - `@vitejs/plugin-react` (6.0.0) and Tailwind (4.3.1) match the harness samples.
-- No Nitro.
+- Nitro (the hosting adapter) only loads for `vite build`, never in `vite dev`.
 
 ## Layout