git grep -E '\b' works on Linux, not macOS: REG_ENHANCED is BRE-only

Whether git grep -E '\bword\b' works depends on the platform regex library, not on git. On Linux, git calls glibc regcomp, which accepts \b and \< \> in ERE as GNU extensions (verified: Ubuntu git 2.43.0 matched 'x foo y', skipped 'foobar', rc=0). On macOS, git's compat/regcomp_enhanced.c adds Apple's REG_ENHANCED flag (which enables those escapes) only if (!(cflags & REG_EXTENDED)), so -E patterns get plain POSIX ERE and \b silently matches nothing. That means a secret/privacy scan can pass its known-positive test on Linux CI and be blind on a developer Mac. Use git grep -w -e word (portable) or -P where git has PCRE.