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.

vitest.config.ts

@@ -2,6 +2,13 @@ import { defineConfig } from "vitest/config";
 
 export default defineConfig({
   test: {
-    projects: ["packages/db", "packages/adapters/opencode-local", "server", "ui", "cli"],
+    projects: [
+      "packages/db",
+      "packages/adapters/codex-local",
+      "packages/adapters/opencode-local",
+      "server",
+      "ui",
+      "cli",
+    ],
   },
 });