dsh-provider-headers: DeepSeek Harness Custom Request Headers Plugin

The DeepSeek Harness model settings page lacks a custom request header editing entry and session ID expansion. This plugin adds a request header editor to the settings page via the plugin mechanism, supporting replacement of `${sessionId}` with the current session ID. It is suitable for scenarios such as OpenCode Go that require specific request headers. The plugin listens to the `llm/stream` waterfall stream and uses `AsyncLocalStorage` to dynamically expand request headers within a session scope. Its scope of effect can be adjusted via a configuration file. Installation requires Node.js and pnpm, and the profile must be restarted after installation. Note that it only overrides the pi-ai route, User-Agent cannot be modified, and request header values are stored in plaintext.