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/tests/invalid-command-handling.test.ts

@@ -10,7 +10,7 @@ describe("rawExec – invalid command handling", () => {
   it("resolves with non‑zero exit code when executable is missing", async () => {
     const cmd = ["definitely-not-a-command-1234567890"];
 
-    const result = await rawExec(cmd, {}, []);
+    const result = await rawExec(cmd, {});
 
     expect(result.exitCode).not.toBe(0);
     expect(result.stderr.length).toBeGreaterThan(0);