Add Rails engine with browser UI at /whetrails

Mounts a Rails engine that lists app models in a sidebar and renders
the full callback/validator chain for any model. Source locations are
linked via vscode:// URIs. Condition badges and concern module tags
distinguish at-a-glance where each callback comes from.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/controllers/whetrails/application_controller.rb

@@ -0,0 +1,23 @@
+module Whetrails
+  class ApplicationController < ActionController::Base
+    layout "whetrails/application"
+    protect_from_forgery with: :exception
+
+    before_action :load_all_models
+
+    private
+
+    def load_all_models
+      ::Rails.application.eager_load!
+      app_root = ::Rails.root.to_s
+      @all_models = ::ActiveRecord::Base.descendants
+        .reject(&:abstract_class?)
+        .reject { |m| m.name.nil? }
+        .select { |m|
+          file = Object.const_source_location(m.name)&.first rescue nil
+          file&.start_with?(app_root)
+        }
+        .sort_by(&:name)
+    end
+  end
+end

app/controllers/whetrails/models_controller.rb

@@ -0,0 +1,16 @@
+module Whetrails
+  class ModelsController < ApplicationController
+    def index
+      if @all_models.any?
+        redirect_to model_path(@all_models.first.name)
+      end
+    end
+
+    def show
+      @model_class = params[:id].constantize
+      @chain = ::Whetrails::Inspector.inspect_model(@model_class)
+    rescue NameError
+      head :not_found
+    end
+  end
+end