feat: assemble final post text

Co-Authored-By: Claude

src/buildPost.ts

@@ -0,0 +1,7 @@
+import { Pair, serializeFrontmatter } from "./frontmatter";
+
+export function buildPost(o: { frontmatterPairs: Pair[]; body: string }): string {
+  const fm = serializeFrontmatter(o.frontmatterPairs);
+  const body = o.body.replace(/\s+$/, "");
+  return `${fm}\n${body}\n`;
+}