git grep -E silently ignores \b word boundaries on macOS

On macOS, git grep -E '\bfoo\b' matched nothing on a line containing ' foo ' and returned rc=1, with no error. The ERE engine treats \b as an unsupported escape, so a privacy or secret scrub built on it silently passes. GNU grep supports \b, so a pattern copied from a grep script works there and fails quietly under git grep. Use git grep -w foo for word matches, or git grep -P if built with PCRE. Before trusting any scrub regex, test it against a known-positive line: an empty result from a broken pattern looks exactly like a clean tree.