fix: Only remove default files when Shakapacker pre-exists

Only remove default bin/dev and config/shakapacker.yml files if
Shakapacker is already configured before running the React on Rails
installer. This prevents us from deleting files that we create ourselves
during the installation process.

The logic now:
- IF Shakapacker already configured → remove default conflicting files
- IF Shakapacker NOT configured → we'll install it, don't remove our own files

This fixes the CI failure where shakapacker.yml was being removed after
we created it, causing subsequent steps to fail.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

lib/generators/react_on_rails/install_generator.rb

@@ -38,7 +38,9 @@ class InstallGenerator < Rails::Generators::Base
 
       def run_generators
         if installation_prerequisites_met? || options.ignore_warnings?
-          remove_default_gem_files
+          # Only remove default files if Shakapacker is already configured
+          # If it's not configured, we'll install it ourselves and shouldn't delete what we create
+          remove_default_gem_files if shakapacker_configured?
           invoke_generators
           add_bin_scripts
           # Only add the post install message if not using Redux