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-server/src/process_sandbox.rs

@@ -185,6 +185,9 @@ pub(crate) async fn prepare_exec_request_with_telemetry(
         network_policy,
     );
     let sandbox_manager = SandboxManager::new();
+    #[cfg(target_os = "macos")]
+    let sandbox_manager = sandbox_manager
+        .with_allowed_symlinked_codex_home(runtime_paths.allowed_symlinked_codex_home.clone());
     let sandbox = sandbox_manager.select_initial(
         &permissions,
         SandboxablePreference::Require,