fix: fail closed on unusable Codex auth during sandbox auth merge

When reusing a Codex sandbox, the workspace-restore step now performs
an auth-merge check before adopting the sandbox's auth.json. The merge
fails closed when:

- host or sandbox Codex auth is unusable (missing token or account)
- the subscription payload shapes differ between host and sandbox
- subscription account identities do not match

Additional hardening: node absence on PATH during the embedded decision
script now exits non-zero instead of silently falling through. Co-change
comments link parseAuth (the embedded script) to hasUsableAuthPayload
(codex-home.ts) so a future auth-format change cannot silently diverge
between the two implementations.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/plugins/examples/plugin-file-browser-example/scripts/build-ui.mjs

@@ -0,0 +1,24 @@
+import esbuild from "esbuild";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+const packageRoot = path.resolve(__dirname, "..");
+
+await esbuild.build({
+  entryPoints: [path.join(packageRoot, "src/ui/index.tsx")],
+  outfile: path.join(packageRoot, "dist/ui/index.js"),
+  bundle: true,
+  format: "esm",
+  platform: "browser",
+  target: ["es2022"],
+  sourcemap: true,
+  external: [
+    "react",
+    "react-dom",
+    "react/jsx-runtime",
+    "@paperclipai/plugin-sdk/ui",
+  ],
+  logLevel: "info",
+});