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/ai/test/xhigh.test.ts

@@ -0,0 +1,69 @@
+import { describe, expect, it } from "vitest";
+import { getModel } from "../src/models.js";
+import { stream } from "../src/stream.js";
+import type { Context, Model } from "../src/types.js";
+
+function makeContext(): Context {
+	return {
+		messages: [
+			{
+				role: "user",
+				content: `What is ${(Math.random() * 100) | 0} + ${(Math.random() * 100) | 0}? Think step by step.`,
+				timestamp: Date.now(),
+			},
+		],
+	};
+}
+
+describe.skipIf(!process.env.OPENAI_API_KEY)("xhigh reasoning", () => {
+	describe("codex-max (supports xhigh)", () => {
+		// Note: codex models only support the responses API, not chat completions
+		it("should work with openai-responses", async () => {
+			const model = getModel("openai", "gpt-5.1-codex-max");
+			const s = stream(model, makeContext(), { reasoningEffort: "xhigh" });
+			let hasThinking = false;
+
+			for await (const event of s) {
+				if (event.type === "thinking_start" || event.type === "thinking_delta") {
+					hasThinking = true;
+				}
+			}
+
+			const response = await s.result();
+			expect(response.stopReason, `Error: ${response.errorMessage}`).toBe("stop");
+			expect(response.content.some((b) => b.type === "text")).toBe(true);
+			expect(hasThinking || response.content.some((b) => b.type === "thinking")).toBe(true);
+		});
+	});
+
+	describe("gpt-5-mini (does not support xhigh)", () => {
+		it("should error with openai-responses when using xhigh", async () => {
+			const model = getModel("openai", "gpt-5-mini");
+			const s = stream(model, makeContext(), { reasoningEffort: "xhigh" });
+
+			for await (const _ of s) {
+				// drain events
+			}
+
+			const response = await s.result();
+			expect(response.stopReason).toBe("error");
+			expect(response.errorMessage).toContain("xhigh");
+		});
+
+		it("should error with openai-completions when using xhigh", async () => {
+			const model: Model<"openai-completions"> = {
+				...getModel("openai", "gpt-5-mini"),
+				api: "openai-completions",
+			};
+			const s = stream(model, makeContext(), { reasoningEffort: "xhigh" });
+
+			for await (const _ of s) {
+				// drain events
+			}
+
+			const response = await s.result();
+			expect(response.stopReason).toBe("error");
+			expect(response.errorMessage).toContain("xhigh");
+		});
+	});
+});