Allow trusted symlinks beneath CODEX_HOME on macOS (#42716)

## What changed

- Add the macOS-only `allow_symlinked_codex_home` user setting. When enabled in the execution host's `$CODEX_HOME/config.toml`, writable roots at or beneath that home may traverse symlinks.
- Keep the exception disabled by default and prevent project config, command-line overrides, and ignored user config from enabling it. Other writable roots remain subject to symlink checks.
- Propagate the resolved setting through local, interactive, and exec-server sandbox paths, including `CODEX_HOME` aliases.
- Explain the opt-out in symlink rejection errors, including that it trusts targets outside `CODEX_HOME` and targets that change between commands.

## Testing

- Add macOS coverage for shell commands, patches, filesystem helpers, process execution, and interactive startup with enabled, disabled, aliased, and out-of-scope homes.

GitOrigin-RevId: 99fcdf611200c9e1b7713cf06f7fdea5bfa7f089

codex-rs/exec/src/lib.rs

@@ -626,6 +626,10 @@ pub async fn run_main(cli: Cli, arg0_paths: Arg0DispatchPaths) -> anyhow::Result
         arg0_paths.codex_self_exe.clone(),
         arg0_paths.codex_linux_sandbox_exe.clone(),
     )?;
+    #[cfg(target_os = "macos")]
+    let local_runtime_paths = local_runtime_paths.with_allowed_symlinked_codex_home(
+        codex_config::allowed_symlinked_codex_home(&config.config_layer_stack, &config.codex_home),
+    );
     let state_db = codex_core::init_state_db(&config).await;
     let environment_manager = if run_loader_overrides.ignore_user_config {
         EnvironmentManager::from_env(Some(local_runtime_paths), config.http_client_factory())