fix: backfill model on gateway sessions after agent runs

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

gateway/session.py

@@ -590,6 +590,7 @@ class SessionStore:
         input_tokens: int = 0,
         output_tokens: int = 0,
         last_prompt_tokens: int = None,
+        model: str = None,
     ) -> None:
         """Update a session's metadata after an interaction."""
         self._ensure_loaded()
@@ -607,7 +608,8 @@ class SessionStore:
             if self._db:
                 try:
                     self._db.update_token_counts(
-                        entry.session_id, input_tokens, output_tokens
+                        entry.session_id, input_tokens, output_tokens,
+                        model=model,
                     )
                 except Exception as e:
                     logger.debug("Session DB operation failed: %s", e)