Skills — SKILL.md, frontmatter, progressive disclosure

Skills package reusable behaviours as a folder containing a SKILL.md file plus
optional supporting scripts and templates. Frontmatter controls invocation: a skill
can be triggered manually, auto-invoked by the model when its description matches
the task, or both. The key technique is progressive disclosure — keep the
SKILL.md lean and move examples, edge cases, and templates into separate files
that are loaded only when relevant, keeping the context window clean. Skills follow
an open standard that works across multiple tools, and modern tooling unifies them
with slash commands so that every skill also exposes a command interface.