dsh-api-mock: Parse OpenAPI to Generate a Zero-Dependency Mock Server

The DeepSeek Harness plugin `dsh-api-mock` addresses the bottleneck of waiting for backend readiness during API integration. It parses OpenAPI 3.x specifications and generates a zero-dependency local mock server. The tool uses built-in Node.js modules, supports deterministic sample data generation, and respects default values in schemas. Developers can call `mock_generate` to start the service or use `mock_sample` to generate test data, enabling earlier frontend development.