Remove Guard

Drop the Guardfile and the guard/guard-rspec/guard-rubocop development
dependencies from the Gemfile.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ericproulx

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 #### Features
 
+* [#2768](https://github.com/ruby-grape/grape/pull/2768): Remove Guard - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 #### Fixes

Gemfile

@@ -16,9 +16,6 @@ end
 group :development do
   gem 'benchmark-ips'
   gem 'benchmark-memory'
-  gem 'guard'
-  gem 'guard-rspec'
-  gem 'guard-rubocop'
   gem 'irb'
 end