Create Context Link

The Docs Context panel's "attach" write path — every OTHER write in this router is purpose-built with a room or chat on one end; nothing creates a link with a document as the context SOURCE. Scoped to sourceType == "document" (the panel's only caller) rather than fully generic, since a fully generic creator would need a visibility check per endpoint type this router doesn't otherwise need. Verifies the caller can see BOTH endpoints before writing — a link row must never itself widen access (PRD §29).