A skill is a folder, not a file. SKILL.md carries the contract; everything else is mounted as-is into the agent workspace, one directory per skill slug, and discovered from the filesystem by the agent runtime. Skills are hierarchical (recursive sub-skills), multi-resource, and sourced from a biome's skills/ folder, the Agent Studio, or a registered git repository. They are owned by skill-registry-api and addressed as XemaObjectKind.Skill. Availability is implicit in ownership: a skill is owned at a space, and the registry declares SKILL_SPACE_KINDS — the admissible subset system, biome, org, project, user over the one seven-member SpaceKind vocabulary — rather than an ownership enum of its own. Two skills at one slug are collapsed by the declared SPACE_KIND_RANK ladder, most-specific winning, and a skill reaches a broader audience by re-scoping (POST /skills/:id/rescope), never through a binding table. See Skills / Authoring for the bundle authoring contract and the resolution model.