Stop configuration discovery at Git worktree boundaries

[vzer200]

When a Git worktree has no local isort configuration, its .git file is ignored during configuration discovery. An outer .isort.cfg can then be inherited, changing the source paths and classifying a local module as third party. Stop discovery at .git files as well as directories, while retaining local config precedence and the existing Mercurial boundary behavior.

Addresses the reproduced outer-configuration case of #2068. This does not establish the cause of every worktree classification problem in that report.

Validation on Windows with Python 3.12:

• Eight regression/control cases: two fail against unchanged main; all pass with the fix. Complete unit suite: 629 passed, 5 skipped, with 99.34% branch-enabled coverage; independently rerun with the same result.
• Actual linked/orphan worktrees and ordinary repositories checked through both the API and CLI. Independent checks also cover separate Git metadata directories, local/empty/unrelated configuration, explicit external settings files and runtime overrides.
• Full Ruff check/format, flake8 and Linux-platform mypy pass. Changed-file isort and Bandit pass. Full checks retain the same baseline findings: Windows os.mkfifo typing, the existing CRLF formatting fixture, and Bandit's assertion finding in parse.py:407.

Other Python/OS runtimes, mypyc builds and the network-cloning integration suite were not run.

AI disclosure: developed and independently reviewed by Codex agents on behalf of vzer200; no human line-by-line review is claimed.