memory_conversations() keeps them in this R session's memory (lost when
it ends): the store a conversation uses when none is named.
folder_store() keeps them in a folder, shared by every process that
opens it (Python's, Julia's and R's on the same folder):
<folder>/conversations/<id>.jsonl (the records, one a line, each append
locked across processes) and <folder>/trees/<turn>.jsonl (each turn's
events, so another process can watch it). Files are readable by their
owner only. A store of your own is a list of append(conversation, records, expect = NULL) and read(conversation, after = 0) functions
(contract/conversations.md, Stores).