refactor(gateway): consolidate model resolution via _resolve_gateway_model()

Replace two inline copies of the env/config model resolution pattern
(in _run_agent_sync and _run_agent) with the _resolve_gateway_model()
helper introduced in PR #830.

Left untouched:
- Session hygiene block: different default (sonnet vs opus) + reads
  compression config from the same YAML load
- /model command: also reads provider from same config block

gateway/run.py

@@ -2029,21 +2029,8 @@ class GatewayRunner:
                 )
                 return
 
-            # Read model from config (same as _run_agent)
+            # Read model from config via shared helper
-            model = os.getenv("HERMES_MODEL") or os.getenv("LLM_MODEL") or "anthropic/claude-opus-4.6"
+            model = _resolve_gateway_model()
-            try:
-                import yaml as _y
-                _cfg_path = _hermes_home / "config.yaml"
-                if _cfg_path.exists():
-                    with open(_cfg_path, encoding="utf-8") as _f:
-                        _cfg = _y.safe_load(_f) or {}
-                    _model_cfg = _cfg.get("model", {})
-                    if isinstance(_model_cfg, str):
-                        model = _model_cfg
-                    elif isinstance(_model_cfg, dict):
-                        model = _model_cfg.get("default", model)
-            except Exception:
-                pass
 
             # Determine toolset (same logic as _run_agent)
             default_toolset_map = {
@@ -3128,21 +3115,7 @@ class GatewayRunner:
             except Exception:
                 pass
 
-            model = os.getenv("HERMES_MODEL") or os.getenv("LLM_MODEL") or "anthropic/claude-opus-4.6"
+            model = _resolve_gateway_model()
-
-            try:
-                import yaml as _y
-                _cfg_path = _hermes_home / "config.yaml"
-                if _cfg_path.exists():
-                    with open(_cfg_path, encoding="utf-8") as _f:
-                        _cfg = _y.safe_load(_f) or {}
-                    _model_cfg = _cfg.get("model", {})
-                    if isinstance(_model_cfg, str):
-                        model = _model_cfg
-                    elif isinstance(_model_cfg, dict):
-                        model = _model_cfg.get("default", model)
-            except Exception:
-                pass
 
             try:
                 runtime_kwargs = _resolve_runtime_agent_kwargs()