refactor(session-log): delete _save_session_log and all callers

state.db now stores every message field the JSON snapshot stored. Removed
the method, all 7 call-sites, and ~13 test stubs that suppressed its file I/O.
Body is in git history if it ever needs to come back.

tests/cron/test_codex_execution_paths.py

@@ -74,7 +74,6 @@ class _Codex401ThenSuccessAgent(run_agent.AIAgent):
         self._cleanup_task_resources = lambda task_id: None
         self._persist_session = lambda messages, history=None: None
         self._save_trajectory = lambda messages, user_message, completed: None
-        self._save_session_log = lambda messages: None
 
     def _try_refresh_codex_client_credentials(self, *, force: bool = True) -> bool:
         type(self).refresh_attempts += 1