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/application.js

@@ -0,0 +1,9 @@
+import { Application } from "@hotwired/stimulus"
+
+const application = Application.start()
+
+// Configure Stimulus development experience
+application.debug = false
+window.Stimulus   = application
+
+export { application }

app/javascript/controllers/hello_controller.js

@@ -0,0 +1,7 @@
+import { Controller } from "@hotwired/stimulus"
+
+export default class extends Controller {
+  connect() {
+    this.element.textContent = "Hello World!"
+  }
+}

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)