fix(models): route bare-provider /model switch through the cost-safe default

`detect_static_provider_for_model` handles a bare provider name typed as a
model (e.g. `/model nous`) by returning `_PROVIDER_MODELS[provider][0]` — the
first curated entry. For metered aggregators whose curated list is ordered
most-capable-first (Nous Portal), entry [0] is the priciest flagship, so
`/model nous` silently switched to it.

This is exactly the billing footgun `_PROVIDER_SILENT_DEFAULT_OVERRIDES` /
`get_default_model_for_provider` exist to prevent (per their docstring, a
missing model "escalated to Opus and billed 863 requests before the user
noticed"). The non-interactive fallback already routes through that cost-safe
helper; the interactive `/model <provider>` path did not.

Route this path through `get_default_model_for_provider` too. Providers
without a silent-default override are unchanged (the helper returns
`models[0]`), so only overridden providers (currently `nous`) change — from
the flagship to the low-cost default.

Adds regression tests: `/model nous` resolves to the cost-safe default, and
non-overridden providers still resolve to their first catalog model.

tests/test_empty_model_fallback.py

@@ -76,6 +76,46 @@ class TestGetDefaultModelForProvider:
             assert result == models_mod._PROVIDER_MODELS["openai-codex"][0]
 
 
+class TestDetectStaticProviderCostSafeDefault:
+    """detect_static_provider_for_model must apply the same cost-safe default
+    as get_default_model_for_provider when a bare provider name is typed as a
+    model (e.g. ``/model nous``)."""
+
+    def test_bare_nous_does_not_escalate_to_flagship(self):
+        from hermes_cli.models import (
+            _PROVIDER_MODELS,
+            get_default_model_for_provider,
+            detect_static_provider_for_model,
+        )
+
+        result = detect_static_provider_for_model("nous", "openrouter")
+        assert result is not None
+        provider, model = result
+        assert provider == "nous"
+        # Must match the cost-safe silent default, NOT the priciest catalog
+        # entry [0]. Regression: this path returned _PROVIDER_MODELS["nous"][0]
+        # directly, re-introducing the billing footgun on the interactive
+        # ``/model nous`` path.
+        assert model == get_default_model_for_provider("nous")
+        assert "opus" not in model.lower()
+        assert model != _PROVIDER_MODELS["nous"][0]
+
+    def test_provider_without_override_still_uses_first_model(self):
+        """Providers with no silent-default override are unchanged."""
+        from hermes_cli.models import (
+            _PROVIDER_MODELS,
+            _PROVIDER_SILENT_DEFAULT_OVERRIDES,
+            detect_static_provider_for_model,
+        )
+
+        for provider in ("anthropic", "xai"):
+            if provider in _PROVIDER_SILENT_DEFAULT_OVERRIDES:
+                continue
+            result = detect_static_provider_for_model(provider, "openrouter")
+            assert result is not None
+            assert result[1] == _PROVIDER_MODELS[provider][0]
+
+
 class TestGatewayEmptyModelFallback:
     """Test that _resolve_session_agent_runtime fills in empty model from provider catalog."""