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.

bin/importmap

@@ -0,0 +1,4 @@
+#!/usr/bin/env ruby
+
+require_relative "../config/application"
+require "importmap/commands"