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

cli.py

@@ -1129,12 +1129,17 @@ class HermesCLI:
         self.verbose = verbose if verbose is not None else (self.tool_progress_mode == "verbose")
         
         # Configuration - priority: CLI args > env vars > config file
-        # Model can come from: CLI arg, LLM_MODEL env, OPENAI_MODEL env (custom endpoint), or config
-        self.model = model or os.getenv("LLM_MODEL") or os.getenv("OPENAI_MODEL") or CLI_CONFIG["model"]["default"]
+        # Model comes from: CLI arg or config.yaml (single source of truth).
+        # LLM_MODEL/OPENAI_MODEL env vars are NOT checked — config.yaml is
+        # authoritative.  This avoids conflicts in multi-agent setups where
+        # env vars would stomp each other.
+        _model_config = CLI_CONFIG.get("model", {})
+        _config_model = _model_config.get("default", "") if isinstance(_model_config, dict) else (_model_config or "")
+        self.model = model or _config_model or "anthropic/claude-opus-4.6"
         # Track whether model was explicitly chosen by the user or fell back
         # to the global default.  Provider-specific normalisation may override
         # the default silently but should warn when overriding an explicit choice.
-        self._model_is_default = not (model or os.getenv("LLM_MODEL") or os.getenv("OPENAI_MODEL"))
+        self._model_is_default = not model
 
         self._explicit_api_key = api_key
         self._explicit_base_url = base_url