Set up Hotwire (importmap, Turbo, Stimulus) and fix bin/ci

The app was generated with 'rails new --skip', which skipped the
JavaScript install step, leaving bin/importmap missing and bin/ci's
importmap audit failing. Run the importmap, turbo, and stimulus install
generators to wire up Hotwire per PRD 10.1/10.2 and restore bin/ci.

app/javascript/application.js

@@ -0,0 +1,3 @@
+// Configure your import map in config/importmap.rb. Read more: https://github.com/rails/importmap-rails
+import "@hotwired/turbo-rails"
+import "controllers"