feat: auto-detect provider when switching models via /model (#1506)

When typing /model deepseek-chat while on a different provider, the
model name now auto-resolves to the correct provider instead of
silently staying on the wrong one and causing API errors.

Detection priority:
1. Direct provider with credentials (e.g. DEEPSEEK_API_KEY set)
2. OpenRouter catalog match with proper slug remapping
3. Direct provider without creds (clear error beats silent failure)

Also adds DeepSeek as a first-class API-key provider — just set
DEEPSEEK_API_KEY and /model deepseek-chat routes directly.

Bare model names get remapped to proper OpenRouter slugs:
  /model gpt-5.4 → openai/gpt-5.4
  /model claude-opus-4.6 → anthropic/claude-opus-4.6

Salvages the concept from PR #1177 by @virtaava with credential
awareness and OpenRouter slug mapping added.

Co-authored-by: virtaava <virtaava@users.noreply.github.com>

tests/hermes_cli/test_models.py

@@ -1,6 +1,6 @@
 """Tests for the hermes_cli models module."""
 
-from hermes_cli.models import OPENROUTER_MODELS, menu_labels, model_ids
+from hermes_cli.models import OPENROUTER_MODELS, menu_labels, model_ids, detect_provider_for_model
 
 
 class TestModelIds:
@@ -54,3 +54,66 @@ class TestOpenRouterModels:
     def test_at_least_5_models(self):
         """Sanity check that the models list hasn't been accidentally truncated."""
         assert len(OPENROUTER_MODELS) >= 5
+
+
+class TestFindOpenrouterSlug:
+    def test_exact_match(self):
+        from hermes_cli.models import _find_openrouter_slug
+        assert _find_openrouter_slug("anthropic/claude-opus-4.6") == "anthropic/claude-opus-4.6"
+
+    def test_bare_name_match(self):
+        from hermes_cli.models import _find_openrouter_slug
+        result = _find_openrouter_slug("claude-opus-4.6")
+        assert result == "anthropic/claude-opus-4.6"
+
+    def test_case_insensitive(self):
+        from hermes_cli.models import _find_openrouter_slug
+        result = _find_openrouter_slug("Anthropic/Claude-Opus-4.6")
+        assert result is not None
+
+    def test_unknown_returns_none(self):
+        from hermes_cli.models import _find_openrouter_slug
+        assert _find_openrouter_slug("totally-fake-model-xyz") is None
+
+
+class TestDetectProviderForModel:
+    def test_anthropic_model_detected(self):
+        """claude-opus-4-6 should resolve to anthropic provider."""
+        result = detect_provider_for_model("claude-opus-4-6", "openai-codex")
+        assert result is not None
+        assert result[0] == "anthropic"
+
+    def test_deepseek_model_detected(self):
+        """deepseek-chat should resolve to deepseek provider."""
+        result = detect_provider_for_model("deepseek-chat", "openai-codex")
+        assert result is not None
+        # Provider is deepseek (direct) or openrouter (fallback) depending on creds
+        assert result[0] in ("deepseek", "openrouter")
+
+    def test_current_provider_model_returns_none(self):
+        """Models belonging to the current provider should not trigger a switch."""
+        assert detect_provider_for_model("gpt-5.3-codex", "openai-codex") is None
+
+    def test_openrouter_slug_match(self):
+        """Models in the OpenRouter catalog should be found."""
+        result = detect_provider_for_model("anthropic/claude-opus-4.6", "openai-codex")
+        assert result is not None
+        assert result[0] == "openrouter"
+        assert result[1] == "anthropic/claude-opus-4.6"
+
+    def test_bare_name_gets_openrouter_slug(self):
+        """Bare model names should get mapped to full OpenRouter slugs."""
+        result = detect_provider_for_model("claude-opus-4.6", "openai-codex")
+        assert result is not None
+        # Should find it on OpenRouter with full slug
+        assert result[1] == "anthropic/claude-opus-4.6"
+
+    def test_unknown_model_returns_none(self):
+        """Completely unknown model names should return None."""
+        assert detect_provider_for_model("nonexistent-model-xyz", "openai-codex") is None
+
+    def test_aggregator_not_suggested(self):
+        """nous/openrouter should never be auto-suggested as target provider."""
+        result = detect_provider_for_model("claude-opus-4-6", "openai-codex")
+        assert result is not None
+        assert result[0] not in ("nous",)  # nous has claude models but shouldn't be suggested