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/tsconfig.json

@@ -0,0 +1,10 @@
+{
+  "extends": "../../../../tsconfig.json",
+  "compilerOptions": {
+    "outDir": "dist",
+    "rootDir": "src",
+    "lib": ["ES2023", "DOM"],
+    "jsx": "react-jsx"
+  },
+  "include": ["src"]
+}