One engine, many selves — the vault stays the same, the persona on top is swappable.
Separate the data from the preferences, rules and history: the vault engine stays the same and the persona loaded on top is swappable.
Your data isn't the thing that changes between contexts — you are. The same trips, transactions, people and notes underlie every version of you, but what the assistant should do with them differs completely depending on whether it's helping you plan a holiday, prepare a client session, or write in your own voice. Rebuilding the data per context is madness; swapping what sits on top of it is trivial.
So the split is clean: the vault is the engine — the facts, the entities, the history, the single source of truth. The persona is a thin layer of preferences, rules, tone and scope loaded over it. One engine, many selves. Change the persona and the same structured data answers a different kind of question, without a byte moving.
It's also the pragmatic answer to sharing. A companion, a colleague or a client gets a persona over a bounded slice — not a copy of your brain, and not a second, drifting version of the truth.