Expose stderr and stdout for commands

Introduce `@stderr` and `@stdout` instance variables to make error
and output streams accessible within commands. This enables more
flexible error handling and message output customization.

Author: aaronmallen

lib/dry/cli.rb

@@ -97,6 +97,10 @@ def call(arguments: ARGV, out: $stdout, err: $stderr)
     # @api private
     def perform_command(arguments)
       command, args = parse(kommand, arguments, [])
+
+      command.instance_variable_set(:@stderr, err)
+      command.instance_variable_set(:@stdout, out)
+
       command.call(**args)
     end
 
@@ -113,6 +117,9 @@ def perform_registry(arguments)
 
       command, args = parse(result.command, result.arguments, result.names)
 
+      command.instance_variable_set(:@stderr, err)
+      command.instance_variable_set(:@stdout, out)
+
       result.before_callbacks.run(command, args)
       command.call(**args)
       result.after_callbacks.run(command, args)

lib/dry/cli/command.rb

@@ -379,6 +379,39 @@ def self.superclass_options
         optional_arguments
         subcommands
       ] => "self.class"
+
+      protected
+
+      # The error output used to print error messaging
+      #
+      # @example
+      #   class MyCommand
+      #     def call
+      #       out.puts "Hello World!"
+      #       exit(0)
+      #     rescue StandardError => e
+      #       err.puts "Uh oh: #{e.message}"
+      #       exit(1)
+      #     end
+      #   end
+      #
+      # @since unreleased
+      # @return [IO]
+      attr_reader :stderr
+
+      # The standard output object used to print messaging
+      #
+      # @example
+      #   class MyCommand
+      #     def call
+      #       out.puts "Hello World!"
+      #       exit(0)
+      #     end
+      #   end
+      #
+      # @since unreleased
+      # @return [IO]
+      attr_reader :stdout
     end
   end
 end