List existing columns with their per-type settings

Use to inspect which columns already exist on this bridge and how they are configured. Returns each column's per-type settings not carried by getBridge (link settings, web-agent model, vendor presence, email generation, webhook URL, cross-bridge cell mirror) plus additionalFields synthetic refs (e.g. {columnId$basis:firstName}) usable in prompts or as referenceFieldPath on createReferenceColumn. Cross-reference entries to the bridge's columns from getBridge by columnId for the base column shape.