A skill directory may hold INSTRUCTIONS.md, with or without SKILL.md. Its body
(frontmatter cut) goes into every system prompt in <skill_instructions>, re-read
on every run: 16 KB per file, 64 KB in total, truncation marked with the file's
path. /skills marks skills with instructions and lists instruction-only dirs.
Skills live in <cwd>/.claude/skills/<name>/SKILL.md (frontmatter name + description).
SkillCatalog re-reads them on every run and the prompt lists name, description and
path; the model reads the full SKILL.md with the read tool when a task matches.
