test(completion): isolate ZshInstaller tests from a real Oh My Zsh install

test/core/completions/installers/zsh-installer.test.ts

@@ -8,15 +8,28 @@ import { ZshInstaller } from '../../../../src/core/completions/installers/zsh-in
 describe('ZshInstaller', () => {
   let testHomeDir: string;
   let installer: ZshInstaller;
+  let originalZsh: string | undefined;
 
   beforeEach(async () => {
+    // Clear $ZSH (set by a real Oh My Zsh install) so isOhMyZshInstalled()
+    // falls through to the isolated test home directory
+    originalZsh = process.env.ZSH;
+    delete process.env.ZSH;
+
     // Create a temporary home directory for testing
     testHomeDir = path.join(os.tmpdir(), `openspec-zsh-test-${randomUUID()}`);
     await fs.mkdir(testHomeDir, { recursive: true });
     installer = new ZshInstaller(testHomeDir);
   });
 
   afterEach(async () => {
+    // Restore original environment
+    if (originalZsh !== undefined) {
+      process.env.ZSH = originalZsh;
+    } else {
+      delete process.env.ZSH;
+    }
+
     // Clean up test directory
     await fs.rm(testHomeDir, { recursive: true, force: true });
   });
@@ -27,6 +40,14 @@ describe('ZshInstaller', () => {
       expect(isInstalled).toBe(false);
     });
 
+    it('should return true when $ZSH environment variable is set', async () => {
+      // No .oh-my-zsh directory in testHomeDir; detection relies on $ZSH alone
+      process.env.ZSH = path.join(testHomeDir, '.oh-my-zsh');
+
+      const isInstalled = await installer.isOhMyZshInstalled();
+      expect(isInstalled).toBe(true);
+    });
+
     it('should return true when Oh My Zsh directory exists', async () => {
       // Create .oh-my-zsh directory
       const ohMyZshPath = path.join(testHomeDir, '.oh-my-zsh');