fix: Isolate tests from the calling git process's environment

[shackmann]

Note

Low Risk
Test-only harness change with no impact on production code or runtime behavior outside the test suite.

Overview
Prevents flaky or destructive test behavior when pytest runs under a git hook (e.g. git commit -a) that exports variables like GIT_INDEX_FILE pointing at this repo’s index instead of a temp repo.

pytest_configure in tests/conftest.py now deletes every GIT_* environment variable before fixtures or xdist workers start. A new test in tests/test_git_environment.py asserts the process sees no GIT_* vars, guarding that isolation.

^{Reviewed by Cursor Bugbot for commit a89eb50. Configure here.}