On Windows, configDir() reads APPDATA, not XDG_CONFIG_HOME. Tests that
only redirected XDG_CONFIG_HOME leaked config state across test files,
causing failures on windows-latest CI. Extract redirectConfigDir/
restoreConfigDir helper that sets both env vars, apply across all 10
test files.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>