Create Seed Skill Tiptap Document

Create the Tiptap document for a seeded starter skill.

Skill documents live at the skill:{skill_id} document name (the convention of features/skills/tiptap/resource.ts tiptapDocumentName), unlike sandbox seed files, whose documents live at the bare file id. Seed skill ids are deterministic (UUIDv5 over org:user:name), so the document name is required to derive from the caller's authenticated organization — a service-key holder cannot materialize documents outside its own org. Creation never overwrites existing content: an existing document returns created: false with already_exists; other Tiptap failures return 502.