Copy default LLM pointers when resolving agents

ResolveAgent aliased the default config's Temperature and MaxTokens
pointers into every agent that left them unset, so all resolved
agents shared one backing value. A mutation through any of those
pointers would corrupt the default and every other agent. It also
dereferenced the default unconditionally even though it can be nil.

Allocate a fresh pointer holding a copy of the default value, and
guard against a nil default so each resolved agent owns independent
state.

Signed-off-by: Émile Ré <emile@probo.com>

pkg/probodconfig/llm_config.go

@@ -96,12 +96,8 @@ func (c *AgentsConfig) ResolveAgent(agent LLMAgentConfig) LLMAgentConfig {
 		agent.ModelName = c.Default.ModelName
 	}
 
-	if agent.Temperature == nil {
-		agent.Temperature = c.Default.Temperature
-	}
-
-	if agent.MaxTokens == nil {
-		agent.MaxTokens = c.Default.MaxTokens
+	if agent.MaxTokens == nil && c.Default.MaxTokens != nil {
+		agent.MaxTokens = new(*c.Default.MaxTokens)
 	}
 
 	return agent