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

.github/workflows/ci.yml

@@ -48,6 +48,10 @@ jobs:
           grep -qv 'PIG Demo — RL environments you can play</title>' dist/demos/wordle/index.html \
             || { echo "demo route kept the homepage title"; exit 1; }
           test -f dist/404.html || { echo "no 404.html"; exit 1; }
+          # sitemap.xml is written by a build step, not by Vite copying public/.
+          # It went missing once because the step was outside `pnpm build`, and
+          # nothing noticed until a 404 on the live host.
+          test -f dist/sitemap.xml || { echo "no sitemap.xml"; exit 1; }
 
       # A blob-backed worker is blocked in production and nowhere else: the
       # site's CSP has no worker-src, so it falls back to default-src 'self'.