fix: preserve nightly backup status and current-branch pushes

[Gujiassh]

Summary

• keep the nightly backup result in the parent shell so the final audit summary reflects the real outcome
• resolve the current branch with git -C instead of relying on a subshell-scoped cd
• push backups to the active branch instead of always targeting main
• fail explicitly when the current branch cannot be determined

Validation

• bash -n scripts/nightly-security-audit.sh

Notes

• clean replacement for the previously closed #3 lane
• refreshed on current origin/main and limited to scripts/nightly-security-audit.sh