refactor: moved temperature method from base provider to model capabilities

docs: added AGENTS.md for onboarding Codex

providers/shared/model_capabilities.py

@@ -1,6 +1,7 @@
 """Dataclass describing the feature set of a model exposed by a provider."""
 
 from dataclasses import dataclass, field
+from typing import Optional
 
 from .provider_type import ProviderType
 from .temperature import RangeTemperatureConstraint, TemperatureConstraint
@@ -33,6 +34,19 @@ class ModelCapabilities:
         default_factory=lambda: RangeTemperatureConstraint(0.0, 2.0, 0.3)
     )
 
+    def get_effective_temperature(self, requested_temperature: float) -> Optional[float]:
+        """Return the temperature that should be sent to the provider.
+
+        Models that do not support temperature return ``None`` so that callers
+        can omit the parameter entirely.  For supported models, the configured
+        constraint clamps the requested value into a provider-safe range.
+        """
+
+        if not self.supports_temperature:
+            return None
+
+        return self.temperature_constraint.get_corrected_value(requested_temperature)
+
     @staticmethod
     def collect_aliases(model_configs: dict[str, "ModelCapabilities"]) -> dict[str, list[str]]:
         """Build a mapping of model name to aliases from capability configs."""