fix(global-config): respect XDG_CONFIG_HOME on all platforms

Prioritize XDG_CONFIG_HOME on Windows to fix test environment overrides.
Previously, Windows would always use APPDATA regardless of XDG_CONFIG_HOME,
causing tests to fail. Now XDG_CONFIG_HOME is checked first on all platforms
before falling back to platform-specific defaults.

Also update the Windows APPDATA test to explicitly clear XDG_CONFIG_HOME
when testing the fallback behavior.

src/core/global-config.ts

@@ -18,10 +18,17 @@ const DEFAULT_CONFIG: GlobalConfig = {
 /**
  * Gets the global configuration directory path following XDG Base Directory Specification.
  *
- * - Unix/macOS: $XDG_CONFIG_HOME/openspec/ or ~/.config/openspec/
- * - Windows: %APPDATA%/openspec/
+ * - All platforms: $XDG_CONFIG_HOME/openspec/ if XDG_CONFIG_HOME is set
+ * - Unix/macOS fallback: ~/.config/openspec/
+ * - Windows fallback: %APPDATA%/openspec/
  */
 export function getGlobalConfigDir(): string {
+  // XDG_CONFIG_HOME takes precedence on all platforms when explicitly set
+  const xdgConfigHome = process.env.XDG_CONFIG_HOME;
+  if (xdgConfigHome) {
+    return path.join(xdgConfigHome, GLOBAL_CONFIG_DIR_NAME);
+  }
+
   const platform = os.platform();
 
   if (platform === 'win32') {
@@ -34,12 +41,7 @@ export function getGlobalConfigDir(): string {
     return path.join(os.homedir(), 'AppData', 'Roaming', GLOBAL_CONFIG_DIR_NAME);
   }
 
-  // Unix/macOS: use XDG_CONFIG_HOME or fallback to ~/.config
-  const xdgConfigHome = process.env.XDG_CONFIG_HOME;
-  if (xdgConfigHome) {
-    return path.join(xdgConfigHome, GLOBAL_CONFIG_DIR_NAME);
-  }
-
+  // Unix/macOS fallback: ~/.config
   return path.join(os.homedir(), '.config', GLOBAL_CONFIG_DIR_NAME);
 }
 

test/core/global-config.test.ts

@@ -70,10 +70,11 @@ describe('global-config', () => {
       }
     });
 
-    it('should use APPDATA on Windows', () => {
+    it('should use APPDATA on Windows when XDG_CONFIG_HOME is not set', () => {
       // This test only makes sense conceptually - we can't change os.platform()
       // But we can verify the APPDATA logic by checking the code path
       if (os.platform() === 'win32') {
+        delete process.env.XDG_CONFIG_HOME;
         const appData = process.env.APPDATA;
         if (appData) {
           const result = getGlobalConfigDir();