Gateway sessions end up with model=NULL because the session row is
created before AIAgent is constructed. After the agent responds,
update_session() writes token counts but never fills in the model.
Thread agent.model through _run_agent()'s return dict into
update_session() → update_token_counts(). The SQL uses
COALESCE(model, ?) so it only fills NULL rows — never overwrites
a model already set at creation time (e.g. CLI sessions).
If the agent falls back to a different provider, agent.model is
updated in-place by _try_activate_fallback(), so the recorded value
reflects whichever model actually produced the response.
Fixes#987
2046a4c08c