dsh-messaging: DeepSeek Harness Multi-Platform Messaging Plugin

`dsh-messaging` is a plugin in the DeepSeek Harness ecosystem for unifying interactions across multiple IM platforms. Through an abstraction layer and the adapter pattern, it shields protocol differences among platforms such as Feishu, Telegram, and Discord, providing a unified API and production-grade features such as automatic Markdown fallback, streaming buffering, message deduplication, and rate-limit retries. After installation and configuration, it can be switched to another platform, but it has limitations: a single instance supports only one platform at a time, Feishu media sending is not implemented, and no test coverage is provided.