test(good-fences): lent a mower, a Think agent mows (red)

The first end-to-end test: a Project Think agent, on the real model,
writes code that runs in a dynamic worker and drives a mower; the test
counts the cells cut, and saves the turn to itest-output/.

Installs @cloudflare/think, @cloudflare/codemode, agents, ai,
workers-ai-provider, zod. The model-test worker now has a main entry,
the Neighbor DO, and a LOADER binding.

Red: src/worker.ts doesn't exist yet.

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

good-fences/itest/save-annotations.ts

@@ -5,7 +5,7 @@ import { mkdir, writeFile } from "node:fs/promises";
 import type { Reporter } from "vitest/reporters";
 
 const outDir = new URL("../itest-output/", import.meta.url);
-const extFor: Record<string, string> = { "text/javascript": "js", "text/plain": "txt" };
+const extFor: Record<string, string> = { "text/javascript": "js", "application/json": "json", "text/plain": "txt" };
 const slug = (text: string) => text.replace(/[^\w]+/g, "-").replace(/^-|-$/g, "");
 
 export default class SaveAnnotations implements Reporter {