gmail-dsh: Native Gmail Integration for DeepSeek Harness

The DeepSeek Harness plugin `gmail-dsh` adopts a plugin-based architecture and registers directly with the DSH system to provide native Gmail access. Its core features include email search, reading, sending (subject to user approval), label management, and status checking. Before performing operations, it requests user permission through an OAuth flow to ensure security. Installation requires configuring and enabling the plugin in the project, obtaining and storing credentials from the Google Cloud Console, and then running the authorization command. During development, attention should be paid to Node module resolution and development-mode configuration. All write operations require user confirmation to ensure the plugin runs securely.