fix(worker): chmod materialized env file to 0600 even when it pre-exists

apps/agent-worker/src/materialize-env.ts

@@ -1,4 +1,4 @@
-import { mkdir, rm, writeFile } from 'node:fs/promises';
+import { chmod, mkdir, rm, writeFile } from 'node:fs/promises';
 import path from 'node:path';
 
 import { assertContainedRealPath } from './path-containment';
@@ -24,6 +24,9 @@ export const writeMaterializedEnv = async (
   });
   await mkdir(path.dirname(envPath), { recursive: true });
   await writeFile(envPath, formatMaterializedEnv(secrets), { mode: 0o600 });
+  // writeFile's mode only applies on create; chmod unconditionally tightens an
+  // already-existing (possibly 0644) file so plaintext secrets never leak.
+  await chmod(envPath, 0o600);
   return envPath;
 };