feat: slug and date derivation

Co-Authored-By: Claude

src/slug.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, test } from "vitest";
+import { slugify, deriveSlug, deriveDate, postFilename } from "./slug";
+
+describe("slugify", () => {
+  test("lowercases, drops apostrophes, hyphenates", () => {
+    expect(slugify("On Making a Game")).toBe("on-making-a-game");
+    expect(slugify("It's a Test!")).toBe("its-a-test");
+    expect(slugify("  Spaced  out  ")).toBe("spaced-out");
+  });
+});
+
+describe("deriveSlug", () => {
+  test("prefers title, falls back to filename", () => {
+    expect(deriveSlug({ title: "Hello World", filename: "note" })).toBe("hello-world");
+    expect(deriveSlug({ title: "  ", filename: "My Note" })).toBe("my-note");
+  });
+});
+
+describe("deriveDate", () => {
+  const now = new Date("2026-06-19T12:00:00Z");
+  test("uses frontmatter Date or string, else now", () => {
+    expect(deriveDate({ frontmatterDate: new Date("2026-02-06T00:00:00Z"), now })).toBe("2026-02-06");
+    expect(deriveDate({ frontmatterDate: "2026-02-18 09:00", now })).toBe("2026-02-18");
+    expect(deriveDate({ now })).toBe("2026-06-19");
+  });
+});
+
+describe("postFilename", () => {
+  test("joins date and slug", () => {
+    expect(postFilename({ date: "2026-06-19", slug: "hello" })).toBe("2026-06-19-hello.md");
+  });
+});