Get node custom agent prompts

This node's custom scenario prompts, which appear alongside the node type's built-in prompts in the Ask-agent dialog. null means the node has never had any set, which is not the same as an empty list. Read separately from the node itself because the list is large enough (50 prompts x 8 KiB per locale) that carrying it on every node listing would be its own problem. Requires read access on the node.