[Imkkey]

Fixes #98.

config_dirs() currently lists ~/.config before an explicit XDG_CONFIG_HOME. If both contain a configuration, the legacy path wins; if neither does, config_dir() creates the new directory under the legacy path. The usage guide describes the XDG location as first.

Place nonempty XDG_CONFIG_HOME first on Unix and macOS, then preserve the existing order of all other candidates. Windows and unset/empty-XDG fallback behavior stay unchanged. Expansion and deduplication continue through the existing code. This follows the narrow ordering approach discussed in #98 and #167; it does not remove legacy lookup paths or add new path validation.

Update the search-path documentation and add subprocess regressions with temporary homes. The cases cover existing configurations, new-directory fallback, empty/unset overrides, duplicate expanded paths, multiple system directories, application-specific overrides, macOS ordering and unchanged Windows paths. The macOS and Windows cases mock platform detection; no native-platform test result is claimed.

Two existing tests needed isolation fixes: FakeHome now saves/restores its temporary HOME/XDG environment, and the explicit-file reload test disables automatic configuration discovery. Without isolation, those tests can consult the machine's real configuration paths.

Validation on Python 3.12.14:

• New tests before the fix: seven failures and 21 passing controls.
• After: all 28 new cases, 44 focused path tests and 327 core tests pass.
• Full suite: 328 passed and one pre-existing release-date failure. A pristine baseline in an isolated environment has 300 passes and the same failure in test_bump_version_applies_sequential_changelog_updates.
• Ruff lint/format, full mypy targeting Python 3.10, strict Sphinx HTML build, documentation formatting/lint and git diff --check pass.

The production change is limited to util.config_dirs(). Pending #213 still obtains candidate paths through that function; its read-only discovery change is independent. The branches were not combined or tested together. Prepared and checked with OpenAI Codex assistance; draft for maintainer review.