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>

whetrails.gemspec

@@ -12,7 +12,7 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = ">= 3.0"
 
-  spec.files         = Dir["lib/**/*.rb", "exe/*", "LICENSE", "README.md"]
+  spec.files         = Dir["lib/**/*.rb", "app/**/*.rb", "app/views/**/*.erb", "config/**/*.rb", "exe/*", "LICENSE", "README.md"]
   spec.executables   = ["whetrails"]
 
   spec.add_dependency "railties", ">= 7.0"