fix: create public output directory for Vercel (framework=null requires it)

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>
Agent-Logs-Url: https://github.com/objectstack-ai/objectql/sessions/e37d14c4-33f2-403e-8745-d5843ed9b0bf

Author: Copilot

apps/demo/public/.gitkeep

@@ -51,4 +51,7 @@ pnpm --filter @objectql/example-project-tracker build
 echo "▸ Patching pnpm symlinks for Vercel…"
 node scripts/patch-symlinks.cjs
 
+# Ensure the output directory exists (Vercel requires it when framework=null)
+mkdir -p public
+
 echo "✓ Vercel build complete."