Replace the Tiptap document of an existing skill row for a starter skill restore (onboarding ADR 0017).

Restore targets keep their row id, and a legacy starter skill row carries a non-deterministic id, so the create route's derivation check cannot authorize this write. Containment comes from the database instead: the skill row must exist under the caller's authenticated organization and the given user, whatever its deleted state. Replacement overwrites an existing document and creates a missing one — a soft-deleted skill's document was deleted with it.