dsh-message-edit: Editing Message Bubbles and Rewriting Context

The `dsh-message-edit` plugin enables users to directly edit any message bubble, rewriting the model-visible context through a `replace` mechanism. Core features include bidirectional editing, Markdown support, Fork inheritance of modified context, and multiple rounds of editing. In terms of implementation, the Host side triggers `surfaceOp: { op: "replace" }` via RPC, while the Client side persists edited content in localStorage and overrides rendering. Relying on DSH core components, the plugin can be installed to provide an on-bubble pencil icon for edits, helping correct errors and improve the conversational experience.