fix: remove unused _writableRoots arg to exec() function (#762)

I suspect this was done originally so that `execForSandbox()` had a
consistent signature for both the `SandboxType.NONE` and
`SandboxType.MACOS_SEATBELT` cases, but that is not really necessary and
turns out to make the upcoming Landlock support a bit more complicated
to implement, so I had Codex remove it and clean up the call sites.

codex-cli/src/utils/agent/sandbox/macos-seatbelt.ts

@@ -72,7 +72,7 @@ export function execWithSeatbelt(
     "--",
     ...cmd,
   ];
-  return exec(fullCommand, opts, writableRoots, abortSignal);
+  return exec(fullCommand, opts, abortSignal);
 }
 
 const READ_ONLY_SEATBELT_POLICY = `