chore(creator-studio): stop tracking the build output

assets/creator-studio/build is generated by `npm run build`, so committing it
put a rebuilt bundle in every diff. It now follows the same deploy-only rule
as assets/admin/build.

- Root .gitignore ignores build/* and node_modules/, keeping a .gitkeep so
  the directory still exists in a clean checkout.
- Drops the three tracked artifacts from the index.
- The contract test no longer requires the artifacts to exist; it checks the
  set is all-or-nothing, so a clean checkout and a locally built tree both
  pass. This mirrors admin-build-graph-contract.
- README documents that build/ is produced on the target machine.

887 widget-system tests pass.

.gitignore

@@ -23,13 +23,19 @@ assets/css/routes/docs/*
 !assets/css/routes/docs/toolbar-search-enhancements.css
 
 # Frontend build output and installed dependencies.
-# assets/admin/build is deploy-only: build it on the target with
+# assets/admin/build and assets/creator-studio/build are deploy-only:
+# build them on the target with
 #   cd assets/admin && npm ci && npm run build
+#   cd assets/creator-studio && npm ci && npm run build
 /assets/admin/node_modules/
 /assets/admin/build/*
 !/assets/admin/build/.gitkeep
 assets/admin/.vite-temp/
 
+/assets/creator-studio/node_modules/
+/assets/creator-studio/build/*
+!/assets/creator-studio/build/.gitkeep
+
 # PHP installed dependencies
 /vendor/*
 !/vendor/.gitkeep