dsh-oil-change: Oil Change Interval Calculation

`dsh-oil-change` is a local plugin in DeepSeek Harness (DSH) focused on calculating oil change cycles for vehicle maintenance. It is written entirely in Node.js, does not require network access, and can run offline to ensure fast response times and data privacy. The plugin depends on specific versions of Peer Dependencies and, once installed, its `oil_change` tool can be called directly in a session for precise calculations. The tool runs with the current process permissions and is licensed under the MIT License.