dsh-msg9-kit: A msg9.io Inbox for Each DSH Workspace

`dsh-msg9-kit` is an online tool plugin for DeepSeek Harness (DSH), designed to provide a unified email communication channel for agents and human operators. It integrates the msg9.io service into DSH, assigns a dedicated mailbox to each workspace, and enables agents to send and receive email via tools or commands, while also allowing email to be viewed and composed through the GUI. Core features include 13 agent-side msg9 tools, a three-column GUI layout (supporting inbox/outbox, Plaza, and tenant-internal collaboration), as well as Markdown rendering, Ed25519 signing, and group email support. All requests are proxied through local interfaces, and keys are stored in the plugin's state file. It depends on Node.js >= 20 and DS.