refactor(cli): Finalize OpenAI Codex Integration with OAuth

- Enhanced Codex model discovery by fetching available models from the API, with fallback to local cache and defaults.
- Updated the context compressor's summary target tokens to 2500 for improved performance.
- Added external credential detection for Codex CLI to streamline authentication.
- Refactored various components to ensure consistent handling of authentication and model selection across the application.

agent/context_compressor.py

@@ -31,7 +31,7 @@ class ContextCompressor:
         threshold_percent: float = 0.85,
         protect_first_n: int = 3,
         protect_last_n: int = 4,
-        summary_target_tokens: int = 500,
+        summary_target_tokens: int = 2500,
         quiet_mode: bool = False,
         summary_model_override: str = None,
     ):