dsh-host-notify: Host-Side macOS Notification Plugin

`dsh-host-notify` is a macOS notification plugin designed specifically for DeepSeek Harness developers. It addresses the issue where notifications from Electron renderer processes are easily blocked by the system when the window is hidden, ensuring reliability by delivering notifications directly through the host process. The plugin supports event listening, foreground activation on click, a deduplication mechanism, and a fallback strategy, and it recommends setting the notification style to "Continuous" in system settings for the best experience.