feat: enhance auxiliary model configuration and environment variable handling

- Added support for auxiliary model overrides in the configuration, allowing users to specify providers and models for vision and web extraction tasks.
- Updated the CLI configuration example to include new auxiliary model settings.
- Enhanced the environment variable mapping in the CLI to accommodate auxiliary model configurations.
- Improved the resolution logic for auxiliary clients to support task-specific provider overrides.
- Updated relevant documentation and comments for clarity on the new features and their usage.

tests/agent/test_auxiliary_client.py

@@ -151,10 +151,10 @@ class TestGetTextAuxiliaryClient:
         assert model is None
 
 
-class TestCodexNotInVisionClient:
-    """Codex fallback should NOT apply to vision tasks."""
+class TestVisionClientFallback:
+    """Vision client uses the same full fallback chain as text."""
 
-    def test_vision_returns_none_without_openrouter_nous(self):
+    def test_vision_returns_none_without_any_credentials(self):
         with patch("agent.auxiliary_client._read_nous_auth", return_value=None):
             client, model = get_vision_auxiliary_client()
         assert client is None