feat(good-fences): complete(messages) on a Workers AI model

makeWorkersAiModel narrows the AI binding to one model's chat
completion: the harness gets `complete`, not `run` on any model at
any cost. gpt-oss-120b replies in the OpenAI shape (checked with a
one-off probe), so the text is choices[0].message.content; anything
else throws, naming the keys it got.

Green: the model counts 3 rs in strawberry.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

good-fences/src/model.ts

@@ -0,0 +1,24 @@
+/** @file The model behind `complete(messages)`: Workers AI, through its binding. */
+
+/** a Workers AI text-generation model with no paid-plan requirement */
+export const MODEL = "@cf/openai/gpt-oss-120b";
+
+export type Message = Readonly<{ role: "system" | "user" | "assistant"; content: string }>;
+
+/** the reply text from an OpenAI-style chat completion */
+const replyText = (output: unknown): string => {
+	const { choices } = (output ?? {}) as { choices?: { message?: { content?: unknown } }[] };
+	const text = choices?.[0]?.message?.content;
+	if (typeof text !== "string") {
+		throw Error(`no reply text in model output with keys: ${Object.keys(output ?? {}).join(", ")}`);
+	}
+	return text;
+};
+
+/** `complete(messages)` on a Workers AI model; `ai` is the AI binding */
+export const makeWorkersAiModel = (ai: Ai, model: string) => ({
+	async complete(messages: readonly Message[]): Promise<string> {
+		const output = await ai.run(model as Parameters<Ai["run"]>[0], { messages: [...messages] });
+		return replyText(output);
+	},
+});