dsh-dejavue: DeepSeek Harness Plugin Duplicate Checker

`dsh-dejaview` is a plugin for DeepSeek Harness, designed to address the problem of developers reinventing the wheel when building new plugins. The plugin registers the `check_plugin_novelty` tool, which retrieves data from the awesome-dsh-plugin registry, GitHub `dsh-plugin` topics, and npm packages, and uses IDF-weighted semantic similarity to score and verify the novelty of plugin ideas. Before building a plugin, an Agent can invoke this tool to obtain candidate plugins, similarity scores, and decision guidance, helping the model make innovation judgments based on objective data and improve development efficiency.