One-time silent migration: if old flat files exist (session.json, identity.json at config root), read session to extract DID, move files into accounts/<did>/, rewrite config with account entry. Run on app startup before any command dispatch.
One-time silent migration: if old flat files exist (session.json, identity.json at config root), read session to extract DID, move files into accounts/<did>/, rewrite config with account entry. Run on app startup before any command dispatch.