Introduce model/ and repository/ structure under lib/

Models: Message, WikiPage, Game (typed structs instead of raw hashes)
Repositories: MessageBoard, OnlineUsers, WikiClient, CatalogClient
bbs.rb uses attribute access (page.title, game.play_path, …) throughout

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/model/message.rb

@@ -0,0 +1,3 @@
+# frozen_string_literal: true
+
+Message = Struct.new(:timestamp, :username, :text)