test(codex-local): regression for CodexRpcClient spawn ENOENT

Add a Vitest case that mocks `node:child_process.spawn` so the child
emits `error` (ENOENT) after the constructor attaches listeners.
`getQuotaWindows()` must resolve with `ok: false` instead of leaving an
unhandled `error` event on the process.

Register `packages/adapters/codex-local` in the root Vitest workspace.

Document in DEVELOPING.md that a missing `codex` binary should not take
down the API server during quota polling.

packages/adapters/codex-local/src/server/quota-spawn-error.test.ts

@@ -0,0 +1,57 @@
+import { EventEmitter } from "node:events";
+import type { ChildProcess } from "node:child_process";
+import { describe, expect, it, vi, beforeEach } from "vitest";
+
+const { mockSpawn } = vi.hoisted(() => ({
+  mockSpawn: vi.fn(),
+}));
+
+vi.mock("node:child_process", async (importOriginal) => {
+  const cp = await importOriginal<typeof import("node:child_process")>();
+  return {
+    ...cp,
+    spawn: (...args: Parameters<typeof cp.spawn>) => mockSpawn(...args) as ReturnType<typeof cp.spawn>,
+  };
+});
+
+import { getQuotaWindows } from "./quota.js";
+
+function createChildThatErrorsOnMicrotask(err: Error): ChildProcess {
+  const child = new EventEmitter() as ChildProcess;
+  const stream = Object.assign(new EventEmitter(), {
+    setEncoding: () => {},
+  });
+  Object.assign(child, {
+    stdout: stream,
+    stderr: Object.assign(new EventEmitter(), { setEncoding: () => {} }),
+    stdin: { write: vi.fn(), end: vi.fn() },
+    kill: vi.fn(),
+  });
+  queueMicrotask(() => {
+    child.emit("error", err);
+  });
+  return child;
+}
+
+describe("CodexRpcClient spawn failures", () => {
+  beforeEach(() => {
+    mockSpawn.mockReset();
+  });
+
+  it("does not crash the process when codex is missing; getQuotaWindows returns ok: false", async () => {
+    const enoent = Object.assign(new Error("spawn codex ENOENT"), {
+      code: "ENOENT",
+      errno: -2,
+      syscall: "spawn codex",
+      path: "codex",
+    });
+    mockSpawn.mockImplementation(() => createChildThatErrorsOnMicrotask(enoent));
+
+    const result = await getQuotaWindows();
+
+    expect(result.ok).toBe(false);
+    expect(result.windows).toEqual([]);
+    expect(result.error).toContain("Codex app-server");
+    expect(result.error).toContain("spawn codex ENOENT");
+  });
+});

packages/adapters/codex-local/vitest.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    environment: "node",
+  },
+});