Promote Harbor Context Draft

Promotes the draft to the next immutable published context version. Promotion publishes the draft's exact title, description, content, and optional change note as the next version. An unpublished context's initial draft becomes version 1. The draft is removed after publication. Promotion returns a conflict when the current published revision differs from the draft's `base_revision_id`. The stale draft remains available so an author can compare it with the current version before discarding and recreating it.