add code exploration fast start

This tries to short-circuit multiple round-trips to llm for reading code.
It's a precursor to trying to context engineer tailored to specific tasks.
In initial experiments, it's only marginally faster than regular mode, and burns more tokens.

crates/g3-providers/src/openai.rs

@@ -384,6 +384,14 @@ impl LLMProvider for OpenAIProvider {
         // OpenAI models support native tool calling
         true
     }
+    
+    fn max_tokens(&self) -> u32 {
+        self.max_tokens.unwrap_or(16000)
+    }
+    
+    fn temperature(&self) -> f32 {
+        self._temperature.unwrap_or(0.1)
+    }
 }
 
 fn convert_messages(messages: &[Message]) -> Vec<serde_json::Value> {