Hooks, skills, subagents, plugins, CLAUDE.md: what each is for

Five ways to extend Claude Code, and the confusion is real: hooks or skills, skills or subagents, CLAUDE.md or a skill. They differ on one axis that decides everything else, when each one enters the context window. Anthropic just published that axis and rebuilt around it. Here is the tested taxonomy, with the token cost of each measured on a clean session.