fix: store last-modified dates in frontmatter (#3002)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bartlomieju

_config.ts

@@ -455,33 +455,11 @@ site.ignore(
 // the default layout if no other layout is specified
 site.data("layout", "doc.tsx");
 
-// Populate lastModified from git history using a single git command
+// Populate lastModified from frontmatter `last_modified` field
 site.preprocess([".md", ".mdx"], (filteredPages) => {
-  const result = Deno.spawnAndWaitSync("git", [
-    "log",
-    "--pretty=format:%aI",
-    "--name-only",
-    "--diff-filter=ACMR",
-    "HEAD",
-  ]);
-
-  const output = new TextDecoder().decode(result.stdout);
-  const lastModified = new Map<string, string>();
-  let currentDate = "";
-
-  for (const line of output.split("\n")) {
-    if (!line) continue;
-    if (line.match(/^\d{4}-/)) {
-      currentDate = line;
-    } else if (!lastModified.has(line)) {
-      lastModified.set(line, currentDate);
-    }
-  }
-
   for (const page of filteredPages) {
-    const src = page.sourcePath?.replace(/^\//, "");
-    if (src && lastModified.has(src)) {
-      page.data.lastModified = new Date(lastModified.get(src)!);
+    if (page.data.last_modified) {
+      page.data.lastModified = new Date(page.data.last_modified);
     }
   }
 });