fix: remove unused import + add corrupt cache test

Address pre-landing review findings:
- Remove unused mkdirSync import from gstack-update-check.test.ts
- Add Path I test: corrupt cache file falls through to remote fetch

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

browse/test/gstack-update-check.test.ts

@@ -7,7 +7,7 @@
  */
 
 import { describe, test, expect, beforeEach, afterEach } from 'bun:test';
-import { mkdtempSync, writeFileSync, rmSync, existsSync, readFileSync, mkdirSync } from 'fs';
+import { mkdtempSync, writeFileSync, rmSync, existsSync, readFileSync } from 'fs';
 import { join } from 'path';
 import { tmpdir } from 'os';
 
@@ -160,6 +160,21 @@ describe('gstack-update-check', () => {
     expect(cache).toContain('UP_TO_DATE');
   });
 
+  // ─── Path I: Corrupt cache file ─────────────────────────────
+  test('falls through to remote fetch when cache is corrupt', () => {
+    writeFileSync(join(gstackDir, 'VERSION'), '0.3.3\n');
+    writeFileSync(join(stateDir, 'last-update-check'), 'garbage data here');
+    // Remote says same version — should end up UP_TO_DATE
+    writeFileSync(join(gstackDir, 'REMOTE_VERSION'), '0.3.3\n');
+
+    const { exitCode, stdout } = run();
+    expect(exitCode).toBe(0);
+    expect(stdout).toBe('');
+    // Cache should be overwritten with valid content
+    const cache = readFileSync(join(stateDir, 'last-update-check'), 'utf-8');
+    expect(cache).toContain('UP_TO_DATE');
+  });
+
   // ─── State dir creation ─────────────────────────────────────
   test('creates state dir if it does not exist', () => {
     const newStateDir = join(stateDir, 'nested', 'dir');