dsh-e2e-dev-sdd: DeepSeek Harness Five-Phase SDD Workbench

The DeepSeek Harness plugin `dsh-e2e-dev-sdd` provides a flexible Spec-Driven Development (SDD) workflow. It decomposes the development process into five optional stages: requirements, prototyping, design, specification, and testing, supporting independent delivery and iteration. The project manages deliverables through a kanban board, uses a Git repository as the source of truth, and combines zero-configuration sources with business adapters to achieve data synchronization. The plugin enforces structural quality checks and a SHA-256 freeze mechanism, ensuring that deliverables remain immutable and traceable. The development stage automatically creates an isolated code space, supports multi-file delivery and version management, and is well suited to building standardized R&D workflows.