refactor: remove LLM_MODEL env var dependency — config.yaml is sole source of truth

Model selection now comes exclusively from config.yaml (set via
'hermes model' or 'hermes setup'). The LLM_MODEL env var is no longer
read or written anywhere in production code.

Why: env vars are per-process/per-user and would conflict in
multi-agent or multi-tenant setups. Config.yaml is file-based and
can be scoped per-user or eventually per-session.

Changes:
- cli.py: Read model from CLI_CONFIG only, not LLM_MODEL/OPENAI_MODEL
- hermes_cli/auth.py: _save_model_choice() no longer writes LLM_MODEL
  to .env
- hermes_cli/setup.py: Remove 12 save_env_value('LLM_MODEL', ...)
  calls from all provider setup flows
- gateway/run.py: Remove LLM_MODEL fallback (HERMES_MODEL still works
  for gateway process runtime)
- cron/scheduler.py: Same
- agent/auxiliary_client.py: Remove LLM_MODEL from custom endpoint
  model detection

hermes_cli/auth.py

@@ -1671,8 +1671,12 @@ def _prompt_model_selection(model_ids: List[str], current_model: str = "") -> Op
 
 
 def _save_model_choice(model_id: str) -> None:
-    """Save the selected model to config.yaml and .env."""
-    from hermes_cli.config import save_config, load_config, save_env_value
+    """Save the selected model to config.yaml (single source of truth).
+
+    The model is stored in config.yaml only — NOT in .env.  This avoids
+    conflicts in multi-agent setups where env vars would stomp each other.
+    """
+    from hermes_cli.config import save_config, load_config
 
     config = load_config()
     # Always use dict format so provider/base_url can be stored alongside
@@ -1681,7 +1685,6 @@ def _save_model_choice(model_id: str) -> None:
     else:
         config["model"] = {"default": model_id}
     save_config(config)
-    save_env_value("LLM_MODEL", model_id)
 
 
 def login_command(args) -> None: