Add CLAUDE.md with guidance for AI coding assistants.
Complements README.md by documenting: this repo is in Perforce (not git),
the two-phase main-playbook architecture (role vs tasks_from), variable
layering precedence, and gotchas (volume path fragments, placeholder
files, plaintext id_rsa, Ubuntu 26.04 sysctl drop-in, sudoers Cmnd_Alias).