Set Default Skill Version Endpoint

Request body for setting a skill's default version.

Shared by POST /v1/skills/{skill_id} (set default version) and UPDATE /v1/skills/{skill_id} (OpenAI-compatible update); both accept ONLY {"default_version": <int>} and repoint the skill's default version.

API response schema for a skill (excludes internal ownership fields).

metadata mirrors the default_version manifest's metadata map so callers can read per-skill flags from the list without an extra request per skill to fetch the version manifest.