fix(hooks): unset GIT_DIR in pre-push so integration tests don't inherit it

[laulauland]

When git push invokes hooks/pre-push, git sets GIT_DIR=.git in the env. That env var leaks into zig build test, and into every subprocess it spawns — including the git commands that integration tests run against temp repos (via helpers.TempRepo). With GIT_DIR set, those subprocess git calls operate on this repo's .git instead of the test's temp directory, which breaks at least one test (check from nested subdir with its own drift.lock only checks that scope expected exit 1, got 0).

The hook passes when run directly but fails under git push. This mismatch hit me while cutting the v0.9.0 release.

Fix: unset GIT_DIR at the top of the hook.