fix(hooks): clear repository git environment for tests

[bmcnaboe]

What does this PR do?

Clears Git's repository-local environment variables only for the pre-commit hook's test subprocess. This keeps staged-index context available to diff-scoped linting while preventing GIT_INDEX_FILE=.git/index from leaking into temporary linked-worktree fixtures.

Root cause

Git exports repository-local variables to hooks. The inherited relative index path works for a normal .git directory but fails when git worktree add checks out into a linked worktree where .git is a file.

Checklist

• Every commit is signed off (git commit -s)
• Tests pass locally (make -f Makefile.cbm test)
• Lint passes (make -f Makefile.cbm lint-ci)
• New behavior is covered by the poisoned-environment reproduction

Validation

• GIT_INDEX_FILE=.git/index ... pipeline before: 249 passed, 1 failed
• isolated environment after: 250 passed
• real pre-commit hook: 7,555 passed, 8 skipped
• scripts/test.sh: 7,555 passed, 0 failed, 8 skipped

Fixes #1740

[github-actions]

Thanks for opening this — it has been seen, and it is queued.

This note is automated, but it is not a brush-off: it exists so you know where your PR stands instead of having to guess from silence.

Current review status: working through a backlog. 0.9.1-rc.1 is out, so the release freeze that held reviews is over — but it left a large queue of open pull requests behind it, and we are reading through them oldest-first. The background is in discussion #1144.

What that means for this PR, concretely:

• It will not be closed for inactivity. No stale bot touches pull requests here.
• It may still sit a while before a human reads it. That is on us, not on you.
• Older PRs are read first, so a recent one is not being skipped — it is behind a queue.

Things that will genuinely speed it up whenever review does happen:

• Keep it rebased on main — the tree is moving quickly right now, and a conflicting branch cannot be reviewed as the diff you intended.
• Get CI green, or say which failures you believe are pre-existing.
• Keep the change to one claim. Bundled features and refactors get split before they get merged, which costs you a round trip.
• Every commit needs a sign-off (git commit -s) — CI enforces DCO.

If this fixes a bug, a reproduction we can run is worth more than a description of the symptom.

Thanks for contributing, and sorry in advance for the wait.