Add xhigh thinking level for OpenAI codex-max models

- Add 'xhigh' to ThinkingLevel type in ai and agent packages
- Map xhigh to reasoning_effort: 'max' for OpenAI providers
- Add thinkingXhigh color token to theme schema and built-in themes
- Show xhigh option only when using codex-max models
- Update CHANGELOG for both ai and coding-agent packages

closes #143

packages/agent/src/transports/types.ts

@@ -7,7 +7,7 @@ export interface AgentRunConfig {
 	systemPrompt: string;
 	tools: AgentTool<any>[];
 	model: Model<any>;
-	reasoning?: "low" | "medium" | "high";
+	reasoning?: "low" | "medium" | "high" | "xhigh";
 	getQueuedMessages?: <T>() => Promise<QueuedMessage<T>[]>;
 }