dsh-tool-diagnose: DeepSeek Harness Real-Time Diagnostics Plugin

`dsh-tool-diagnose` is a plugin for DeepSeek Harness (DSH) designed to eliminate the need for developers to manually run commands or read configurations when checking Cordis state, such as plugins, tools, and credentials. Through in-process introspection, it exposes a `diagnose` tool to the model, providing real-time inspection of plugin status, tool visibility, credential parsing, approval policies, token pressure, and the subagent tree. The plugin supports installation in Web UI and Headless environments and includes a `target` parameter to filter the scope of checks. It also introduces an extensible mechanism based on the `ctx.diagnostics` registry, allowing third-party plugins to easily register custom checks and addressing the limitation of traditional Skills serving merely as a "symptom table."