Fetch raw SKILL.md content for a published skill

Returns the raw SKILL.md body as text/markdown. This is the ONLY content-delivery route in the catalog that does not wrap the payload in a JSON envelope — clients must set their Accept header accordingly (or accept the default).

404 is emitted BOTH when the skill does not exist and when it exists but has no SKILL.md content attached — the error body differs (Skill not found vs Content not available), but clients should treat both as terminal.