Move Skills out of CLAUDE.md so reference material costs zero tokens until invoked

Anthropic Engineering·medium signal

When you keep pasting the same playbook into chat, convert it into a Skill. Unlike CLAUDE.md content (which loads into every turn), a Skill's body loads only when the model invokes it — letting you keep large reference docs available without paying their token cost on every request. This is the cleanest lever for shrinking always-on context.