pre-commit hook leaks GIT_INDEX_FILE into linked-worktree tests #1740

Description

With core.hooksPath=scripts/hooks, running git commit reaches the pre-commit hook's full test step but fails git_context_linked_worktree.

Reproduction

1. Configure the repository hook: git config core.hooksPath scripts/hooks.
2. Stage any change and run git commit.
3. Observe the sequential test failure:

git_context_linked_worktree
FAIL tests/test_pipeline.c:6855: run_cmd(cmd) == 32768, expected 0 == 0

The failure can be reproduced without a commit:

GIT_INDEX_FILE=.git/index build/c/test-runner pipeline

This yields 249 passed and 1 failed. The same pipeline suite passes 250/250 when the repository-local Git environment is absent.

Root cause

Git exports repository-local variables such as GIT_INDEX_FILE=.git/index to hooks. The test runner inherits that value. It works while a fixture uses a normal .git directory, but git worktree add checks out into a linked worktree where .git is a file, so the inherited .git/index path is invalid.

Expected behavior

The hook should preserve Git's staged-index environment for diff-scoped linting, then clear repository-local Git variables only for the test subprocess.

Validation

After isolating the test subprocess environment:

• poisoned-environment pipeline regression: 250 passed
• real pre-commit hook: 7,555 passed, 8 skipped
• canonical scripts/test.sh: 7,555 passed, 0 failed, 8 skipped