fix: exclude bundled API handler from git (build artifact)

Agent-Logs-Url: https://github.com/objectstack-ai/framework/sessions/b62bdefa-1175-4e27-b791-d526cae03b4b

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Claude

examples/app-host/.gitignore

@@ -0,0 +1,19 @@
+# Build artifacts
+dist/
+.turbo/
+
+# Bundled API handler (generated during Vercel build)
+api/_handler.js
+api/_handler.js.map
+
+# Node modules
+node_modules/
+
+# Environment files
+.env
+.env.local
+.env.*.local
+
+# OS files
+.DS_Store
+Thumbs.db