Single-source the version from package.json (closes #5)

src/index.ts imports package.json for VERSION and bin/quak.ts passes
VERSION to commander, so package.json is the only place the version is
written. tsc copies package.json to dist/package.json, so the import
resolves from the built output; script/build runs the built CLI with
--version to prove it. A test checks VERSION and quak --version against
package.json.

Model: opus-5-5

test/smoke.test.ts

@@ -1,9 +1,43 @@
-import { describe, expect, it } from "vitest";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { readFileSync } from "node:fs";
 import { VERSION } from "../src/index.js";
 
-describe("quak", () => {
-    it("exports a version string", () => {
-        expect(typeof VERSION).toBe("string");
-        expect(VERSION.length).toBeGreaterThan(0);
+const packageVersion = (
+    JSON.parse(
+        readFileSync(new URL("../package.json", import.meta.url), "utf-8"),
+    ) as { version: string }
+).version;
+
+class ExitCalled extends Error {}
+
+describe("version", () => {
+    const argv = process.argv;
+
+    afterEach(() => {
+        process.argv = argv;
+        vi.restoreAllMocks();
+    });
+
+    it("exports the version from package.json", () => {
+        expect(VERSION).toBe(packageVersion);
+    });
+
+    // Runs bin/quak.ts with --version. commander prints the version and then
+    // calls process.exit, which is stubbed to throw so the test survives.
+    it("reports the version from package.json in quak --version", async () => {
+        const printed: string[] = [];
+        vi.spyOn(process.stdout, "write").mockImplementation((chunk) => {
+            printed.push(String(chunk));
+            return true;
+        });
+        vi.spyOn(process, "exit").mockImplementation(() => {
+            throw new ExitCalled();
+        });
+        process.argv = ["node", "quak", "--version"];
+
+        await expect(import("../bin/quak.js")).rejects.toBeInstanceOf(
+            ExitCalled,
+        );
+        expect(printed.join("").trim()).toBe(packageVersion);
     });
 });