listFields discovers field IDs for customFields inputs and dynamic mappings.
automations/list-apollo-fields.automation.ts
source is "crm_synced" | "custom" | "system"; omitting it returns
all sources. Optional account selects the binding. Returns { fields } with
fieldId, label, modality, type, contexts, source, metadata, workspace,
and timestamps. Custom-field mutation inputs are keyed by fieldId, not label.