fullstack-expert: DeepSeek Harness's Cordis-Native Full-Stack Engineering Workflow

`fullstack-expert` is a Cordis-native plugin for DeepSeek Harness, designed to address the lack of engineering workflows and traceable evidence in complex agent tasks. It extends Harness with native capabilities, providing features such as repository checks, vertical-slice planning, sensitive-operation boundaries, and evidence logging. Without introducing additional components, the plugin standardizes full-stack engineering workflows by registering native skills and permission approvals, ensuring transparent and controllable operations for users seeking rigorous development processes.