Playbook Package authoring skeleton

Copy this directory to content/playbooks/<slug>/, rename playbook.yaml.template to playbook.yaml and replace every ${...} authoring token. Rename the other .template files to their final names and declare them in package.files.

The template is deliberately not accepted as runtime content. A completed package must validate against schemas/playbook.schema.json, include no unresolved authoring tokens and pass python3 scripts/validate_pack.py.