feat: assemble final post text

Co-Authored-By: Claude

src/buildPost.test.ts

@@ -0,0 +1,14 @@
+import { expect, test } from "vitest";
+import { buildPost } from "./buildPost";
+
+test("assembles frontmatter + body with single trailing newline", () => {
+  const out = buildPost({
+    frontmatterPairs: [{ key: "layout", value: "post" }],
+    body: "Hello world",
+  });
+  expect(out).toBe("---\nlayout: post\n---\n\nHello world\n");
+});
+
+test("empty frontmatter still emits a delimiter block", () => {
+  expect(buildPost({ frontmatterPairs: [], body: "x" })).toBe("---\n\n---\n\nx\n");
+});