xby-sequentialthinking: Sequential Thinking Service Plugin for DeepSeek Harness

`xby-sequentialthinking` is a workflow plugin for DeepSeek Harness that addresses complex multi-step reasoning problems through a sequential thinking protocol. Its core features include configuring an API key and the `sequentialthinking` tool, which supports iterative reasoning, path backtracking, and hypothesis validation. Users must first obtain a key, then flexibly adjust the number of thinking steps, challenge decisions, or explore branches within the workflow until the termination condition is met. This plugin is suitable for complex planning or design tasks, enabling dynamic optimization of solutions.