Print CLI errors as one line instead of a stack trace (closes #102)

An error a command throws now reaches the user as one `quak: MESSAGE`
line on stderr, and the CLI exits 1 once output has drained. The wrapper
moved from bin/quak.ts to src/cli-run.ts so it can be tested, and
bin/quak.ts awaits program.parseAsync() so async actions are awaited.

Model: opus-5-5

test/cli/run.test.ts

@@ -0,0 +1,58 @@
+/**
+ * Tests for `run` in `src/cli-run.ts`, which every CLI command goes through.
+ */
+
+import { PassThrough } from "node:stream";
+import { describe, it, expect } from "vitest";
+
+import { run } from "../../src/cli-run.js";
+
+// A stream whose written text is kept in `text`; writes finish at once, so
+// nothing is left waiting to drain.
+const collector = (): { stream: PassThrough; text: () => string } => {
+    const stream = new PassThrough();
+    const chunks: string[] = [];
+    stream.on("data", (chunk: Buffer) => chunks.push(chunk.toString()));
+    return { stream, text: () => chunks.join("") };
+};
+
+const runToExit = async (
+    command: Promise<number>,
+): Promise<{ code: number; stdout: string; stderr: string }> => {
+    const stdout = collector();
+    const stderr = collector();
+    const code = await new Promise<number>((resolve) => {
+        void run(command, stdout.stream, stderr.stream, resolve);
+    });
+    return { code, stdout: stdout.text(), stderr: stderr.text() };
+};
+
+describe("run", () => {
+    it("exits with the code the command returns", async () => {
+        const result = await runToExit(Promise.resolve(3));
+        expect(result).toEqual({ code: 3, stdout: "", stderr: "" });
+    });
+
+    it("prints a thrown error as one line without a stack trace and exits 1", async () => {
+        const failing = async (): Promise<number> => {
+            throw new Error(
+                "ENOTDIR: not a directory, mkdir '/dev/null/x/originals'",
+            );
+        };
+        const result = await runToExit(failing());
+        expect(result.code).toBe(1);
+        expect(result.stdout).toBe("");
+        expect(result.stderr).toBe(
+            "quak: ENOTDIR: not a directory, mkdir '/dev/null/x/originals'\n",
+        );
+    });
+
+    it("prints a thrown value that is not an Error", async () => {
+        const result = await runToExit(Promise.reject("offline"));
+        expect(result).toEqual({
+            code: 1,
+            stdout: "",
+            stderr: "quak: offline\n",
+        });
+    });
+});