dsh-callout: Convert DSH Assistant Markdown to GitHub-Style Callout Cards

The `dsh-callout` plugin for DeepSeek Harness (DSH) converts Markdown alert markers (e.g., `> [!NOTE]`) into GitHub-style semantic callout cards. It supports five types—NOTE, TIP, IMPORTANT, WARNING, and CAUTION—with theme-aware styling and support for nesting multiple Markdown elements within cards. The plugin only transforms styles on the frontend without modifying the source content, and implements edit fallback through the `dsh.plugin` observer. Install it with `dsh plugin --profile web add github:Aafff623/dsh-callout`; a hard refresh of the Web UI is required for it to take effect.