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>

config/application.rb

@@ -14,7 +14,8 @@ module Whetrails
     # Please, add to the `ignore` list any other `lib` subdirectories that do
     # not contain `.rb` files, or that should not be reloaded or eager loaded.
     # Common ones are `templates`, `generators`, or `middleware`, for example.
-    config.autoload_lib(ignore: %w[assets tasks])
+    # lib/whetrails/**/* uses manual requires (gem source), not Zeitwerk autoloading
+    config.autoload_lib(ignore: %w[assets tasks whetrails.rb whetrails])
 
     # Configuration for the application, engines, and railties goes here.
     #