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/controllers/index.js

@@ -0,0 +1,4 @@
+// Import and register all your controllers from the importmap via controllers/**/*_controller
+import { application } from "controllers/application"
+import { eagerLoadControllersFrom } from "@hotwired/stimulus-loading"
+eagerLoadControllersFrom("controllers", application)