dsh-llm-key-rotation: Automatically rotate keys and retry when DeepSeek Harness quota is exhausted

Introduction: If you have been running the same LLM provider in DeepSeek Harness (DSH) for a long time, you have likely encountered this situation: a task is interrupted halfway, the request returns QUOTA or 429, the entire workflow is forced to stop, requiring manual key replacement, process restart, and context restoration. A common approach in the community is to write scripts to monitor logs and perform manual intervention, but such solutions are detached from Harness's own request flow and cannot handle the task of "retrying immediately after switching keys". The `dsh-llm-key-rotation` plugin written by m1khal3v integrates this step into Harness's recovery chain: it automatically switches to the next key in the configuration chain and retries immediately upon request failure, without the need to restart.