fix(security): Address review on git-config hardening

fix(git-untrusted-config): put the diff hardening flags after the caller's options so a passed --ext-diff/--textconv can never re-enable an external driver (git honors the last of conflicting flags); no current caller does, but the invariant should hold unconditionally
fix(git-hardening-test): restrict the real-git end-to-end block to POSIX — the payload is a /bin/sh script and gpg.program is an absolute path whose Windows backslashes git's config parser reads as escapes, so it would fail on the windows-latest CI leg; the cross-platform guarantee stays covered by the exact-argument assertions
learned(git-untrusted-config): repomix CI runs the vitest suite on ubuntu, windows and macos, so real-git disk-execution tests must guard for platform

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>