Message header shows the producing assistant/agent first, model is secondary

What changed

Each message now displays the assistant (chat) or agent (session) that produced it as the primary avatar + name in the header, with the model demoted to a small muted secondary label. The producing author and model are frozen onto the message at send time, so a renamed or deleted assistant/agent/model still shows its original identity on past messages.

Why this matters to the user

The header previously led with the model. Existing conversations will now read "<Assistant name> · <model>" instead of model-first, and historical messages keep the author/model they were generated with even after the live entity is renamed or removed.

What the user should do

Nothing — automatic. Newly sent messages capture the snapshot; older messages without one fall back to their stored model id.

Notes for release manager

Purely presentational + per-message metadata; no data loss.

v1-imported chat messages do get an author snapshot when the topic's assistant and the message's model resolve during migration; only rows where that can't be resolved stay snapshot-less.

Snapshot-less rows resolve their model from the row's own stored modelId. The message header uses no live-model fallback, so switching the default model or the assistant/agent never moves a past message's header. A row with neither a snapshot nor a modelId simply shows no model label. (Session export still fills such rows with the current agent model as a last resort, since an export is a one-shot snapshot rather than a live-updating header.)