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/raw-exec.ts

@@ -20,7 +20,6 @@ import * as os from "os";
 export function exec(
   command: Array<string>,
   options: SpawnOptions,
-  _writableRoots: ReadonlyArray<string>,
   abortSignal?: AbortSignal,
 ): Promise<ExecResult> {
   // Adapt command for the current platform (e.g., convert 'ls' to 'dir' on Windows)