Clear Repository-Location Variables in Every Temporary-Repository Test Through One Helper #3324

Defect

A test or self-test that builds a temporary repository with git -C <tmp> or cwd=<tmp> inherits any GIT_DIR, GIT_WORK_TREE, or GIT_INDEX_FILE in its environment. A git hook sets these, so a suite run from a hook writes its init, config, and commits into the hook's own repository instead of the temporary one.

#3323 fixes this inside the write guard's --selftest, which clears the repository-location variables once around every case. The guard installs as one standalone file, so it cannot share that primitive, and the other sites carry their own differing lists or none:

• spec/audit.py _selftest: its temporary repositories (the deletion probe and the cases after it) run git init, git config user.email, git add, and git commit with cwd= and no clearing at all. The file has no GIT_DIR handling anywhere, so this is the destructive case.
• host-setup/agent-safety/claude/test_install.py, TestAliasReadThroughLinkedWorktreeGitfile: drops GIT_DIR, GIT_WORK_TREE, GIT_COMMON_DIR, and GIT_OBJECT_DIRECTORY, but not GIT_INDEX_FILE, which a pre-commit hook sets, before its commit and worktree add.
• tests/test_pr_review.py _DISCOVERY_VARS / _clean_env, tests/test_pre_commit_hook.py (strips by the GIT_ prefix), and scripts/local_review.py INHERITED_REDIRECTS (three names): each a different variant of the same list.

Reproduction

Construct a scratch repository, point GIT_DIR at its .git, and run python3 spec/audit.py --selftest. The scratch repository's config and history change.

Expected

One shared helper for the scripts and tests that can import it, holding the full repository-location list, with every site above moved onto it. spec/audit.py's self-test comes first, since it clears nothing.