Add demo:publish npm script for one-shot redeploy

Wraps the build + rsync into a single command for the routine
edit-and-ship loop.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -21,6 +21,7 @@
     "chromatic": "chromatic --exit-zero-on-changes --build-script-name=build:storybook",
     "demo:dev": "vite -c vite.demo.config.ts",
     "demo:build": "vite build -c vite.demo.config.ts",
+    "demo:publish": "npm run demo:build -- --mode arrangement && ./scripts/deploy-demo.sh arrangement",
     "prepare": "husky"
   },
   "dependencies": {