dsh-site-audit: A health check for the site before launch

`dsh-site-audit` is a DSH plugin that performs a read-only health check after site generation and before deployment. It checks internal links for 4xx/5xx status codes and timeouts, oversized images, basic SEO components, the presence of `robots.txt` and `sitemap.xml`, mixed content, and excessively long redirect chains. It supports online crawling (`--base`) and local directory scanning (`--dir ./dist`, which sends no requests), outputs block/warning/pass reports, and supports `--json` and `--out report.md`. The exit codes (0/1/2) allow it to be directly integrated into deployment scripts as a gatekeeper. The plugin only sends GET and HEAD requests; it does not submit forms, log in, or transmit data externally.