seo: sitemap lastmod uses the licence date for entries and is missing for the home and freshness pages #108

Why it matters
Crawlers use lastmod to decide what to refetch. Entry pages change every weekly rebuild (age strings, "used in" links, pager) but report the verified date, so a page that changed last week claims it changed in July. The home page and /freshness/, which change weekly by design, have no lastmod at all.

Evidence

• site/build.mjs:305-308.

Suggested approach
Use the build date for the home and freshness URLs, and for entries the later of verified and the entry file's last commit date (git log -1 --format=%cs -- <file>, cached per build), falling back to the build date when git is unavailable.

Done when
Every sitemap URL has a lastmod that is never earlier than the last content change.