Rename postsPath to defaultPath, remove date requirement from postFilename

src/slug.test.ts

@@ -29,4 +29,8 @@ describe("postFilename", () => {
   test("joins date and slug", () => {
     expect(postFilename({ date: "2026-06-19", slug: "hello" })).toBe("2026-06-19-hello.md");
   });
+
+  test("renders correctly without date", () => {
+    expect(postFilename({ slug: "hello-without-date" })).toBe("hello-without-date.md");
+  });
 });