Create Harbor Context Draft

Creates an unpublished context document with its initial mutable draft. Creates a stable context identity and its initial mutable draft without publishing content to the Harbor. The response includes `context_id`, which identifies the draft replacement, promotion, and discard routes. The initial draft has a null `base_revision_id`. It remains absent from normal context list/detail, GraphQL, entity lookup, and Harbor MCP reads until promoted. Use the regular context creation endpoint instead when the initial payload should be published immediately.