dsh-fert-plan: Plant Care Plan Plugin

`dsh-fert-plan` is a plugin for DeepSeek Harness (DSH) designed to provide agents with plant care plans through local computation. It is implemented in pure Node.js, has no network dependencies, and registers a `fert_plan` tool for agents to call. Before use, install the required dependencies and ensure your DSH version meets the requirements. It is suitable for plant care scenarios that do not rely on real-time data, and a security note recommends reviewing the source code.