full-core.gembox: exclude mruby-sleep from gembox

Since full-core.gembox includes mruby-task (which defines
MRB_USE_TASK_SCHEDULER), mruby-sleep's implementation becomes disabled
via conditional compilation. Exclude mruby-sleep from full-core.gembox
to avoid loading an effectively empty gem. mruby-task provides
task-aware sleep/usleep implementations instead.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/full-core.gembox

@@ -1,6 +1,6 @@
 MRuby::GemBox.new do |conf|
   Dir.glob("#{root}/mrbgems/mruby-*/mrbgem.rake") do |x|
     g = File.basename(File.dirname(x))
-    conf.gem :core => g unless g =~ /^mruby-(?:bin-debugger|test)$/
+    conf.gem :core => g unless g =~ /^mruby-(?:bin-debugger|test|sleep)$/
   end
 end