Fix RuboCop config for development environment

[taketo1113]

This Pull Request fixes an error that occurs when running RuboCop in the development environment.

Expected behavior

$ bundle exec rubocop
Inspecting 29 files
.............................

29 files inspected, no offenses detected

Actual behavior

$ ruby -v
ruby 3.4.8 (2025-12-17 revision 995b59f666) +PRISM [arm64-darwin25]
$ bundle exec rubocop -V
1.81.7 (using Parser 3.3.10.0, rubocop-ast 1.48.0, analyzing as Ruby 3.0, running on ruby 3.4.8) [arm64-darwin25]

$ bundle exec rubocop
Unable to find gem rubocop-discourse; is the gem installed? Could not find 'rubocop-discourse' (>= 0) among 123 total gem(s)
Checked in 'GEM_PATH=/Users/taketo/path/slim-rails/tmp/ruby/3.4.0' , execute `gem env` for more information
/Users/taketo/path/slim-rails/tmp/ruby/3.4.0/gems/rubocop-1.81.7/lib/rubocop/config_loader_resolver.rb:310:in 'RuboCop::ConfigLoaderResolver#gem_config_path'
...