Durable session-locator row: external id -> upstream identity. A pointer, not an event ledger.
@type status() :: :active | :terminated | :deleted
@type t() :: %ReqManagedAgents.Host.Locator.Record{ agent_id: String.t() | nil, context_sent: boolean(), environment_id: String.t() | nil, external_id: String.t(), metadata: map(), session_id: String.t() | nil, status: status() }
@spec new( String.t(), keyword() ) :: t()