dsh-scratchpad: Shared Floating Text Editor

`dsh-scratchpad` is a plugin for DeepSeek Harness (DSH) that provides a floating text editor, addressing the lack of UI for displaying and editing large blocks of text. It exists as a low-level service with no persistent storage; text is only valid within the current session. The plugin supports two modes: Read (read-only) and Edit (editable). In Edit mode, it supports Save, Cancel, and keyboard shortcut operations, and clicking the background overlay does not close it to prevent accidental actions. When using it, the service must be injected and its `open` method called, with an `onSave` callback provided in Edit mode. Additionally, it relies on `dsh-diff-view` to render Diff content, and the maximum text size is limited to 2MB.