Account Export

Hand the owner their data. A GET because it is a read and because a download is what "export" means: the archive is built in memory and streamed back, not written into a server-side directory the owner can never reach (which is what the old POST did, behind a button that fetched a /v1/export that never existed). X-Membase-Export-Memory lets the UI say immediately whether the agent's memory made it in, without opening the archive.