Add prettier (closes #1)

Install prettier globally beside wrangler and wrangler's peers, and
check it in the smoke test. prettier ships no distro package for
Ubuntu, so npm global is the standard install.

Model: deepseek-v4-pro

bin/test

@@ -20,6 +20,7 @@ docker run --rm "$img" bash -ec '
     go version; node --version; python3 --version; hugo version | head -1
     COREPACK_ENABLE_NETWORK=0 yarn --version
     COREPACK_ENABLE_NETWORK=0 pnpm --version
+    prettier --version
 '
 
 echo "== $img: runit service lifecycle"