Third-party dsh web plugin

Key features

• Third-party DeepSeek Harness web plugin: a rich Markdown notebook for the composer region.
• Its browser half contributes two entries to the composer region that dsh-client-ui-conversation owns: a tool-row toggle (conversation.input.left) and an editor card in the composer context stack (conversation.input.dock).
• Both entries share one per-session store handle, so the in-progress draft survives closing and reopening the panel, surface remounts, and — because the engine scopes session stores by session id — each session keeps its own notebook.
• The editor is a CodeMirror 6 instance over the Markdown language: syntax highlighting for the draft (GFM tables included), native undo/redo and selection, and Enter-key list editing in the Codex style — pressing Enter on a non-empty list item opens the next item (ordered markers increment 1.
• → 2., checkbox items reopen unchecked, indentation is preserved, and a mid-line caret splits the item), while Enter on an empty item drops the marker and returns the line to plain-text editing.