feat(scripts): #24 SEO audit agent — crawl + structural checks + markdown report (#76)

`scripts/agents/seo-audit.ts` crawls a running marketing instance (default
http://localhost:3000) up to `--depth 3`, runs cheap structural checks on
every page, and writes `docs/seo-audit-YYYY-MM-DD.md` grouped by URL.

Checks: HTTP 200, title 30..65, meta description 50..160, single <h1>,
<img alt>, canonical, JSON-LD schema.org, robots.txt, sitemap.xml.

Lighthouse is intentionally out of scope here — note in script comment.

Closes #24

Author: f-amine