Tee conversation store: every WRITE is persisted to the wrapped backing store (so history reads stay correct) and ALSO published to the bus as a `ConversationCaptured` event, so the transcript stream flows through the same event pipeline as feedback and observability. The producer (the `SessionGrain`) keeps depending only on `IConversationStore`; swapping the backing store for a database/cloud implementation needs no producer change, and any subscriber can persist or forward the events independently.