dsh-message-copy-enhance: Enhanced Markdown Copy for DeepSeek Harness Assistant Messages

The `dsh-message-copy-enhance` plugin is designed to address formatting loss when copying DeepSeek Harness (DSH) assistant messages into Markdown editors. By intercepting copy events, it converts rendered HTML into complete Markdown while preserving links, LaTeX source, code block languages, and GitHub-style elements such as tables and lists. It can be installed with `dsh plugin --profile desktop add dsh-message-copy-enhance`. Note that the plugin only intercepts selections within an individual assistant message and cannot handle cross-message selections; some special links and KaTeX elements that fail to render may not be fully restored.