test(utils): add unit tests for systemd file handling

- Add tests for writing .service and .timer files with `writeUnitFiles`
- Add tests for resolving target paths with `resolveUnitTargetPath`
- Ensure proper file creation, content validation, and path resolution

src/utils/fs.ts

@@ -0,0 +1,30 @@
+import { ensureDir } from 'https://deno.land/std@0.224.0/fs/mod.ts';
+import { join } from 'https://deno.land/std@0.224.0/path/mod.ts';
+import { TimerOptions } from '../types/options.ts';
+
+export async function writeUnitFiles(
+    name: string,
+    serviceContent: string,
+    timerContent: string,
+    options: TimerOptions,
+): Promise<{ servicePath: string; timerPath: string }> {
+    const basePath = resolveUnitTargetPath(options);
+
+    await ensureDir(basePath);
+
+    const servicePath = join(basePath, `${name}.service`);
+    const timerPath = join(basePath, `${name}.timer`);
+
+    await Deno.writeTextFile(servicePath, serviceContent);
+    await Deno.writeTextFile(timerPath, timerContent);
+
+    return { servicePath, timerPath };
+}
+
+export function resolveUnitTargetPath(
+    options: Pick<TimerOptions, 'output' | 'user'>,
+): string {
+    if (options.output) return options.output;
+    if (options.user) return `${Deno.env.get('HOME')}/.config/systemd/user`;
+    return '/etc/systemd/system';
+}