dsh-explorer-plugin: Add workspace file browser and Monaco editor to DSH Web GUI

dsh-explorer-plugin is a plugin developed for the DeepSeek Harness (DSH) Web GUI, designed to enable users to manage files within the session workspace directly from the sidebar without switching to an external editor. The plugin provides file tree browsing capabilities, supporting file operations such as creation, renaming, moving, and deletion, and integrates the Monaco editor, offering features like multi-tabs, undo/redo, syntax highlighting (supporting 28 languages based on VS Code TextMate grammar), and external conflict detection. Functionally, the plugin monitors file changes on the agent side in real-time via SSE and automatically updates the file tree. It provides shortcut actions for "Analyze" and "Fix," which send instructions to the agent upon user confirmation. Regarding security, all operations are strictly confined to the session workspace.