Add CleanedOutput class

This class is responsible for taking the output from an external command
and cleaning it up ready to be included in a GitHub comment.

lib/cleaned_output.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+require 'colorize'
+
+class CleanedOutput
+  def initialize(output:)
+    @output = output
+  end
+
+  def to_s
+    cleaned_output[-64_000..-1] || cleaned_output
+  end
+
+  private
+
+  def cleaned_output
+    output.gsub(morph_api_key, 'REDACTED').uncolorize
+  end
+
+  def morph_api_key
+    ERB::Util.url_encode(ENV['MORPH_API_KEY'])
+  end
+
+  attr_reader :output
+end

test/cleaned_output_test.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+require 'test_helper'
+
+describe 'CleanedOutput' do
+  describe 'basic output' do
+    subject { CleanedOutput.new(output: 'Hello, world') }
+
+    it 'returns it untouched' do
+      subject.to_s.must_equal 'Hello, world'
+    end
+  end
+
+  describe 'output containing Morph API key' do
+    before do
+      @old_key = ENV['MORPH_API_KEY']
+      ENV['MORPH_API_KEY'] = 'test-morph-api-key'
+    end
+
+    after { ENV['MORPH_API_KEY'] = @old_key }
+
+    subject { CleanedOutput.new(output: 'Key: test-morph-api-key') }
+
+    it 'removes the key from the output' do
+      subject.to_s.must_equal 'Key: REDACTED'
+    end
+  end
+
+  describe 'with color escape codes in output' do
+    let(:output) { 'Test' }
+    subject { CleanedOutput.new(output: output) }
+
+    it 'strips escape codes' do
+      subject.to_s.must_equal 'Test'
+    end
+  end
+
+  describe 'large output' do
+    let(:output) { 'x' * 100_000 }
+    subject { CleanedOutput.new(output: output) }
+
+    it 'only returns that last 64k' do
+      subject.to_s.size.must_equal 64_000
+    end
+  end
+end