sitemap.xml was 404 on the live host; make the build own it

sitemap.mjs was a step I ran by hand, so the moment I rebuilt without it the
file vanished from dist and the deploy shipped a site with no sitemap — on a
site whose entire point is being findable. Nothing caught it because nothing
was looking.

It is now inside `pnpm build`, CI asserts the file exists, and deploy.sh
smoke-tests /sitemap.xml, /robots.txt and an OG card against the live hostname.
The 404 assertion was also a warning rather than a failure; it exits now.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019mt6sHQHEnEYrJZvoMCJSB

package.json

@@ -10,7 +10,7 @@
   "packageManager": "pnpm@11.21.0",
   "scripts": {
     "dev": "vite",
-    "build": "tsc --noEmit && vite build && node scripts/prerender.mjs",
+    "build": "tsc --noEmit && vite build && node scripts/prerender.mjs && node scripts/sitemap.mjs",
     "preview": "vite preview --port 4173",
     "typecheck": "tsc --noEmit",
     "test": "tsx --test src/demos/*/__tests__/*.test.ts",