Add bundler/gem_tasks so rake build exists for the release workflow

The shared gem_release.yml workflow's Build step runs `bundle exec
rake build`, but this Rakefile never defined a build task -- it only
had spec/changelog. First real release attempt failed immediately:
"Don't know how to build task 'build'". require 'bundler/gem_tasks'
provides the standard build/install/release tasks; no new dependency
needed since bundler is already required for everything.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: actowery

Rakefile

@@ -1,3 +1,4 @@
+require "bundler/gem_tasks"
 require "rspec/core/rake_task"
 
 RSpec::Core::RakeTask.new(:spec)