feat: add GPT-5-Codex support with Responses API integration

Adds support for OpenAI's GPT-5-Codex model which uses the new Responses API
endpoint (/v1/responses) instead of the standard Chat Completions API.

Changes:
- Add GPT-5-Codex to MODEL_CAPABILITIES with 400K context, 128K output
- Prioritize GPT-5-Codex for EXTENDED_REASONING tasks
- Add aliases: codex, gpt5-codex, gpt-5-code
- Update tests to expect GPT-5-Codex for extended reasoning

Benefits:
- 40-80% cost savings through Responses API caching
- 3% better performance on coding tasks (SWE-bench)
- Leverages existing dual-API infrastructure

tests/test_auto_mode_comprehensive.py

@@ -93,7 +93,7 @@ class TestAutoModeComprehensive:
                     "OPENROUTER_API_KEY": None,
                 },
                 {
-                    "EXTENDED_REASONING": "o3",  # O3 for deep reasoning
+                    "EXTENDED_REASONING": "gpt-5-codex",  # GPT-5-Codex prioritized for coding tasks
                     "FAST_RESPONSE": "gpt-5",  # Prefer gpt-5 for speed
                     "BALANCED": "gpt-5",  # Prefer gpt-5 for balanced
                 },