refactor: use ENOTDIR approach for cross-platform install error tests

Instead of platform-specific invalid paths (Z:\ or /root), create a
temporary file and use it as homeDir. This guarantees deterministic
ENOTDIR failures when trying to create subdirectories on all platforms.

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

@@ -139,12 +139,11 @@ describe('BashInstaller', () => {
     });
 
     it('should handle installation errors gracefully', async () => {
-      // Create installer with non-existent/invalid home directory
-      // Use a path that will fail on both Unix and Windows
-      const invalidPath = process.platform === 'win32'
-        ? 'Z:\\nonexistent\\invalid\\path'  // Non-existent drive letter on Windows
-        : '/root/invalid/nonexistent/path';  // Permission-denied path on Unix
-      const invalidInstaller = new BashInstaller(invalidPath);
+      // Create a temporary file and use its path as homeDir
+      // This guarantees ENOTDIR when trying to create subdirectories (cross-platform)
+      const blockingFile = path.join(testHomeDir, 'blocking-file');
+      await fs.writeFile(blockingFile, 'blocking content');
+      const invalidInstaller = new BashInstaller(blockingFile);
 
       const result = await invalidInstaller.install(testScript);
 
@@ -377,12 +376,11 @@ describe('BashInstaller', () => {
     });
 
     it('should handle write permission errors gracefully', async () => {
-      // Create installer with path that can't be written
-      // Use a path that will fail on both Unix and Windows
-      const invalidPath = process.platform === 'win32'
-        ? 'Z:\\nonexistent\\invalid\\path'  // Non-existent drive letter on Windows
-        : '/root/invalid/path';  // Permission-denied path on Unix
-      const invalidInstaller = new BashInstaller(invalidPath);
+      // Create a temporary file and use its path as homeDir
+      // This guarantees ENOTDIR when trying to write .bashrc (cross-platform)
+      const blockingFile = path.join(testHomeDir, 'blocking-file');
+      await fs.writeFile(blockingFile, 'blocking content');
+      const invalidInstaller = new BashInstaller(blockingFile);
 
       const result = await invalidInstaller.configureBashrc(completionsDir);
 

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

@@ -286,7 +286,9 @@ complete -c openspec -a 'init'
       expect(result.message).toBe('Completion script uninstalled successfully');
     });
 
-    it('should return failure on permission error', async () => {
+    // Skip on Windows: fs.chmod() on directories doesn't restrict write access on Windows
+    // Windows uses ACLs which Node.js chmod doesn't control
+    it.skipIf(process.platform === 'win32')('should return failure on permission error', async () => {
       await installer.install(mockCompletionScript);
       const targetPath = path.join(testHomeDir, '.config', 'fish', 'completions', 'openspec.fish');
       const parentDir = path.dirname(targetPath);

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

@@ -171,7 +171,9 @@ describe('PowerShellInstaller', () => {
       expect(content).toContain('Write-Host "Hello"');
     });
 
-    it('should skip configuration when script line already exists', async () => {
+    // Skip on Windows: Windows has dual profile paths (PowerShell Core + Windows PowerShell 5.1),
+    // so even if one profile is already configured, the second one will be configured and return true
+    it.skipIf(process.platform === 'win32')('should skip configuration when script line already exists', async () => {
       delete process.env.OPENSPEC_NO_AUTO_CONFIG;
       const profilePath = installer.getProfilePath();
       await fs.mkdir(path.dirname(profilePath), { recursive: true });