feat(agent): allow declaring supports_vision via user config

Custom/local provider models absent from models.dev get classified as
non-vision and have their image content stripped before reaching the
upstream API. Surface a user-facing override:

  model:
    supports_vision: true

  providers:
    my-vllm:
      models:
        my-llava:
          supports_vision: true

The override short-circuits the models.dev lookup in
_model_supports_vision(), which is the single gate guarding image-strip
preprocessing on every transport path.

Refs #8731.

tests/run_agent/test_vision_aware_preprocessing.py

@@ -168,3 +168,28 @@ class TestModelSupportsVision:
         agent = _make_agent()
         with patch("agent.models_dev.get_model_capabilities", side_effect=RuntimeError("boom")):
             assert agent._model_supports_vision() is False
+
+    def test_top_level_model_override_wins(self):
+        agent = _make_agent()
+        agent.provider = "custom"
+        agent.model = "my-llava"
+        with patch("hermes_cli.config.load_config", return_value={"model": {"supports_vision": True}}), \
+             patch("agent.models_dev.get_model_capabilities", return_value=None):
+            assert agent._model_supports_vision() is True
+
+    def test_per_provider_per_model_override_wins(self):
+        agent = _make_agent()
+        agent.provider = "custom"
+        agent.model = "my-llava"
+        cfg = {"providers": {"custom": {"models": {"my-llava": {"supports_vision": True}}}}}
+        with patch("hermes_cli.config.load_config", return_value=cfg), \
+             patch("agent.models_dev.get_model_capabilities", return_value=None):
+            assert agent._model_supports_vision() is True
+
+    def test_override_false_disables_vision_for_models_dev_models(self):
+        agent = _make_agent()
+        fake_caps = MagicMock()
+        fake_caps.supports_vision = True
+        with patch("hermes_cli.config.load_config", return_value={"model": {"supports_vision": False}}), \
+             patch("agent.models_dev.get_model_capabilities", return_value=fake_caps):
+            assert agent._model_supports_vision() is False