dsh-a11y-scan: Native Accessibility Scanning Tool for DeepSeek Harness

`dsh-a11y-scan` is a native DSH plugin that automates frontend accessibility compliance (WCAG) testing using the `axe-core` engine. It supports scanning local files, build artifacts, or URLs and generates structured reports in Markdown, HTML, or JSON, flagging `incomplete` items that require manual review. The tool operates in read-only mode and can run as a standalone utility in CI/CD pipelines, effectively improving the efficiency of accessibility reviews.