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>

.github/scripts/check-pr-lockfile.mjs

@@ -0,0 +1,31 @@
+#!/usr/bin/env node
+/**
+ * check-pr-lockfile.mjs
+ * Checks that pnpm-lock.yaml was not manually edited.
+ * Export: checkLockfile(files, prAuthor, prBranch) → { passed, failures }
+ */
+import { fileURLToPath } from 'node:url';
+
+export function checkLockfile(files, prAuthor, prBranch) {
+  const lockfileChanged = files.some(f => f.filename === 'pnpm-lock.yaml');
+  if (!lockfileChanged) return { passed: true, failures: [] };
+
+  const isRefreshBot =
+    prAuthor === 'github-actions[bot]' && prBranch === 'chore/refresh-lockfile';
+
+  return {
+    passed: isRefreshBot,
+    failures: isRefreshBot ? [] : [
+      'You have changes to `pnpm-lock.yaml` — `pr.yml` will hard-fail this PR with a confusing message about lockfile edits. ' +
+      'To fix: run `pnpm install` locally, exclude the lockfile from your commit, push again. ' +
+      'The lockfile is regenerated automatically by the refresh bot on a schedule.',
+    ],
+  };
+}
+
+if (process.argv[1] === fileURLToPath(import.meta.url)) {
+  const files = JSON.parse(process.env.PR_FILES ?? '[]');
+  const result = checkLockfile(files, process.env.PR_AUTHOR ?? '', process.env.PR_BRANCH ?? '');
+  console.log(JSON.stringify(result));
+  process.exit(result.passed ? 0 : 1);
+}