Function fixed_git_hook_path 

pub fn fixed_git_hook_path(
    project_root: &Path,
    tool_type_dir: &str,
    event_filename: &str,
) -> Option<PathBuf>

Tier-1: Check whether a fixed-path git hook override file exists.

Returns the override file path if it exists, or None to fall through.

tool_type_dir is the directory name under hooks/ (e.g., "git", "prek-with-agent"). event_filename is the git hook filename (e.g., "pre-commit", "commit-msg").