feat: add toolbox contract shell and testkit

examples/react-app/README.md

@@ -0,0 +1,15 @@
+# React integration example
+
+The source files show the intended integration points for an existing React
+application. Copy `public/toolbox-app.json` into the build unchanged (or inject
+the package version during the build), import the shell CSS once, and wrap the
+application in `AppShell`.
+
+In the built application, run:
+
+```sh
+toolbox-check dist
+```
+
+The adjacent example catalog is illustrative. A production catalog normally
+lives at the toolbox home and points to each deployed app manifest.