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/mower.ts

@@ -13,3 +13,38 @@ export const aboutMower = {
   stop(): Promise<void>;
 }`,
 };
+
+const headings = [
+	[0, -1], // north
+	[1, 0], // east
+	[0, 1], // south
+	[-1, 0], // west
+] as const;
+
+/** a mower on a `width` × `height` yard, starting at `at`; pushing past an
+ * edge wraps to the other side (6d) */
+export const makeMower = ({ width, height, at }: { width: number; height: number; at: { x: number; y: number } }) => {
+	let { x, y } = at;
+	let heading = 0;
+	const cut = new Set<string>();
+	const cutHere = () => cut.add(`${x},${y}`);
+	return {
+		start() {
+			heading = 0;
+			cutHere();
+		},
+		push() {
+			const [dx, dy] = headings[heading];
+			x = (x + dx + width) % width;
+			y = (y + dy + height) % height;
+			cutHere();
+		},
+		turn() {
+			heading = (heading + 1) % headings.length;
+		},
+		stop() {},
+		cutCells: () => [...cut],
+	};
+};
+
+export type Mower = ReturnType<typeof makeMower>;