feat(good-fences): a Think agent mows with a codemode mower connector

Neighbor is a Project Think agent on gpt-oss-120b. Its execute tool
runs model-written code in a dynamic worker; MowerConnector puts the
mower there as the `mower.*` namespace (start/push/turn/stop, each
taking an args object). makeMower is the Logo-turtle mower on a
wrapping yard; for now it lives inside the agent's DO.

Getting to green, one symptom at a time:
- chatted instead of acting: say it acts by running code
- reply cut mid-sentence: maxOutputTokens 4096 via beforeTurn
- execute failed creating codemode's runtime facet ("Incorrect type
  for the 'class' field"): declare CodemodeRuntime as a DO class
- looked for the mower in the workspace filesystem: activeTools
  ["execute"], and name the namespace in the prompt
- found the mower, then asked for lawn dimensions: "don't ask me
  questions: put it to use"

Green: 36 mower calls, a 30-cell rectangle.

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

good-fences/src/neighbor.ts

@@ -0,0 +1,44 @@
+/** @file The neighbor's harness as a Project Think agent, lent a mower. */
+import { Think } from "@cloudflare/think";
+import { createExecuteTool } from "@cloudflare/think/tools/execute";
+import { createWorkersAI } from "workers-ai-provider";
+
+import { MAX_TOKENS, MODEL } from "./model.ts";
+import { MowerConnector } from "./mower-connector.ts";
+import { makeMower } from "./mower.ts";
+
+type Env = { AI: Ai; LOADER: WorkerLoader };
+
+export class Neighbor extends Think<Env> {
+	#mower = makeMower({ width: 40, height: 40, at: { x: 20, y: 20 } });
+
+	getModel() {
+		return createWorkersAI({ binding: this.env.AI })(MODEL);
+	}
+
+	getSystemPrompt() {
+		return `You are my neighbor. You have 50 acres of lawn that need mowing.
+You act by running code with the execute tool. Things I lend you are
+namespaces there: the mower is \`mower.*\`. Don't ask me questions: when I
+lend you something, put it to use, then tell me what you did.`;
+	}
+
+	beforeTurn() {
+		// only code: Think's workspace tools sent the model looking for the
+		// mower in the filesystem
+		return { maxOutputTokens: MAX_TOKENS, activeTools: ["execute"] };
+	}
+
+	getTools() {
+		return {
+			execute: createExecuteTool(this, {
+				connectors: [new MowerConnector(this.ctx, this.env, this.#mower)],
+			}),
+		};
+	}
+
+	/** the cells the mower has cut, as `"x,y"` */
+	cutCells() {
+		return this.#mower.cutCells();
+	}
+}