Instructions that always load are expensive. Skills are how you keep a procedure on the shelf until it is needed.
Skill
Also called Agent Skill, SKILL.md
A folder holding a SKILL.md file: a name, a one-line description and steps to follow, sometimes with scripts beside it. Only the name and description load at first. The agent reads the rest when the description matches what it is doing.
You can keep fifty procedures available for roughly the price of fifty one-liners. That makes the description the part that matters most: write it vaguely and the skill never fires, and you will think the agent ignored you.
“Turn our release steps into a skill at .claude/skills/release/SKILL.md. Description: "Use when cutting a release or bumping the version." Keep it under 100 lines and put the changelog script next to it.”