feat: Codex OAuth vision support + multimodal content adapter

The Codex Responses API (chatgpt.com/backend-api/codex) supports
vision via gpt-5.3-codex. This was verified with real API calls
using image analysis.

Changes to _CodexCompletionsAdapter:
- Added _convert_content_for_responses() to translate chat.completions
  multimodal format to Responses API format:
  - {type: 'text'} → {type: 'input_text'}
  - {type: 'image_url', image_url: {url: '...'}} → {type: 'input_image', image_url: '...'}
- Fixed: removed 'stream' from resp_kwargs (responses.stream() handles it)
- Fixed: removed max_output_tokens and temperature (Codex endpoint rejects them)

Provider changes:
- Added 'codex' as explicit auxiliary provider option
- Vision auto-fallback now includes Codex (OpenRouter → Nous → Codex)
  since gpt-5.3-codex supports multimodal input
- Updated docs with Codex OAuth examples

Tested with real Codex OAuth token + ~/.hermes/image2.png — confirmed
working end-to-end through the full adapter pipeline.

Tests: 2459 passed.

tests/agent/test_auxiliary_client.py

@@ -167,12 +167,14 @@ class TestVisionClientFallback:
         assert client is None
         assert model is None
 
-    def test_vision_auto_skips_codex(self, codex_auth_dir):
-        """Even with Codex available, vision auto mode returns None (Codex can't do multimodal)."""
-        with patch("agent.auxiliary_client._read_nous_auth", return_value=None):
+    def test_vision_auto_includes_codex(self, codex_auth_dir):
+        """Codex supports vision (gpt-5.3-codex), so auto mode should use it."""
+        with patch("agent.auxiliary_client._read_nous_auth", return_value=None), \
+             patch("agent.auxiliary_client.OpenAI"):
             client, model = get_vision_auxiliary_client()
-        assert client is None
-        assert model is None
+        from agent.auxiliary_client import CodexAuxiliaryClient
+        assert isinstance(client, CodexAuxiliaryClient)
+        assert model == "gpt-5.3-codex"
 
     def test_vision_auto_skips_custom_endpoint(self, monkeypatch):
         """Custom endpoint is skipped in vision auto mode."""