test(hooks): cover pre-commit file size guard

[taigrr]

Summary

• add regression coverage for the pre-commit large-file guard
• verify deleted staged files still pass through the hook
• run the new pre-commit regression test in ShellCheck CI

Tests

• shellcheck pre-commit commit-msg pre-push post-checkout post-commit post-merge install.sh uninstall.sh test/install_test.sh test/pre_commit_test.sh
• bash test/install_test.sh
• bash test/pre_commit_test.sh
• git diff --check