fix(seo): use git commit dates for dateModified instead of build date

Replace new Date().toISOString() with git log timestamps so
dateModified in TechArticle JSON-LD reflects actual content changes
rather than claiming every page was modified on each deploy.

Author: cursoragent

website/src/layouts/DocsPage.astro

@@ -7,6 +7,7 @@ interface Props {
 }
 
 import BaseLayout from "./BaseLayout.astro";
+import { execSync } from "node:child_process";
 
 const props = Astro.props;
 const title = props.title ?? props.frontmatter?.title ?? '';
@@ -36,6 +37,20 @@ const siteOrigin = "https://useprimer.dev";
 const pagePath = Astro.url.pathname.replace(/\/$/, "") || "/";
 const pageUrl = `${siteOrigin}${pagePath}/`;
 
+function getGitDateModified(pathname: string): string {
+  try {
+    const slug = pathname.replace(/^\//, "").replace(/\/$/, "");
+    const gitRoot = execSync("git rev-parse --show-toplevel", { encoding: "utf-8" }).trim();
+    const candidates = [`${gitRoot}/website/src/pages/${slug}.mdx`, `${gitRoot}/website/src/pages/${slug}.md`, `${gitRoot}/website/src/pages/${slug}.astro`];
+    for (const filePath of candidates) {
+      const ts = execSync(`git log -1 --format=%aI -- "${filePath}"`, { encoding: "utf-8" }).trim();
+      if (ts) return ts.split("T")[0];
+    }
+  } catch { /* ignore */ }
+  return new Date().toISOString().split("T")[0];
+}
+const dateModified = getGitDateModified(pagePath);
+
 // Two-level breadcrumb (Documentation → page). We don't include the
 // frontmatter `section` tier because the docs index uses different
 // section groupings than the frontmatter, so there's no real landing
@@ -48,7 +63,7 @@ const structuredData = [
     headline: title,
     description,
     inLanguage: "en",
-    dateModified: new Date().toISOString().split("T")[0],
+    dateModified,
     author: { "@type": "Organization", name: "Primer", url: siteOrigin },
     isPartOf: { "@type": "WebSite", name: "Primer Documentation", url: `${siteOrigin}/docs/` },
     mainEntityOfPage: { "@type": "WebPage", "@id": pageUrl },