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/helpers/whetrails/models_helper.rb

@@ -0,0 +1,20 @@
+module Whetrails
+  module ModelsHelper
+    def vscode_uri(file, line)
+      "vscode://file/#{file}:#{line}" if file
+    end
+
+    def short_path(file)
+      return nil unless file
+      file.sub("#{::Rails.root}/", "")
+    end
+
+    def condition_label(c)
+      c.is_a?(Symbol) ? ":#{c}" : "<proc>"
+    end
+
+    def validator_type_label(type)
+      type.split("::").last.sub("Validator", "").downcase
+    end
+  end
+end