feat(good-fences): code-mode prompt; the model writes code to mow

codeModeMessages turns what I say plus `about` into a prompt: a
neighbor persona, rules for the reply (one async arrow function over
`env`), and the lent things' types. Things are remote and calls on one
are delivered in order, so a lamp example shows pipelining: await only
for a value, or on a batch's last call. With only the rule, the model
awaited every call; with the example, it pipelines.

aboutMower gives the Mower type with Promise returns. The model test
checks the code calls only Mower methods, pushes, and awaits less often
than it calls; it saves the code to itest-output/ via a reporter, since
the test runs in workerd, which can't write files. Model tests get 60s.

Green: both model tests.

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

good-fences/src/mower.ts

@@ -0,0 +1,15 @@
+/** @file The mower: Logo-turtle moves on a yard of cells. */
+
+/** what a model is told about a mower: its type, with the moves explained */
+export const aboutMower = {
+	ts: `interface Mower {
+  /** put the mower down, facing north, and cut the cell under it */
+  start(): Promise<void>;
+  /** move forward one cell (10 yards), cutting the cell it enters */
+  push(): Promise<void>;
+  /** turn right 90 degrees, in place */
+  turn(): Promise<void>;
+  /** park the mower */
+  stop(): Promise<void>;
+}`,
+};