New script setup-linux-admin-user.sh creates the Linux system user,
installs an SSH authorized key, and adds the user to the sudo group.
Integrated into bootstrap.sh before harden-ssh.sh so key-based access
is in place before password authentication is disabled.
bootstrap.sh now accepts ADMIN_USER and ADMIN_SSH_KEY env vars to run
user setup and setup-admin-sudo.sh automatically at the right point.
audit.sh checks that at least one non-root user has an authorized key.
docs/04-security-hardening.md updated with the new steps and ordering.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
