dsh-decision-split: Split-screen reading plugin for DSH Web UI

`dsh-decision-split` is a DSH Web UI plugin maintained by yuanbaoerer, designed to address the issue of Agent messages becoming excessively long and occupying the main window's context. Following the "everything is a plugin" philosophy, the plugin provides a sidebar reading access point for text-based Agent messages in completed rounds. Its core functionality is to display a "View Full Text in Sidebar" button at the end of messages. Upon clicking, the full Markdown content (supporting tables, code blocks, etc.) is displayed and scrolled independently in a right-side detail panel, with the first heading automatically extracted as the panel title. The sidebar has a default width of 480px and supports collapsing and switching via clicking the button again, closing via the top-right corner, or clicking other message buttons. This plugin does not include an embedded input box or decision feedback functionality; decision interactions still rely on the native `ask_user_question`.