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>

Gemfile.lock

@@ -1,3 +1,10 @@
+PATH
+  remote: .
+  specs:
+    whetrails (0.1.0)
+      activerecord (>= 7.0)
+      railties (>= 7.0)
+
 GEM
   remote: https://rubygems.org/
   specs:
@@ -420,6 +427,7 @@ DEPENDENCIES
   turbo-rails
   tzinfo-data
   web-console
+  whetrails!
 
 BUNDLED WITH
    2.5.22