feat: Add support for --add-dir to exec and TypeScript SDK (#6565)

## Summary

Adds support for specifying additional directories in the TypeScript SDK
through a new `additionalDirectories` option in `ThreadOptions`.

## Changes

- Added `additionalDirectories` parameter to `ThreadOptions` interface
- Updated `CodexExec` to accept and pass through additional directories
via the `--config` flag for `sandbox_workspace_write.writable_roots`
- Added comprehensive test coverage for the new functionality

## Test plan

- Added test case that verifies `additionalDirectories` is correctly
passed as repeated flags
- Existing tests continue to pass

---------

Co-authored-by: Claude <noreply@anthropic.com>

codex-rs/exec/tests/suite/add_dir.rs

@@ -0,0 +1,72 @@
+#![cfg(not(target_os = "windows"))]
+#![allow(clippy::expect_used, clippy::unwrap_used)]
+
+use core_test_support::responses;
+use core_test_support::test_codex_exec::test_codex_exec;
+
+/// Verify that the --add-dir flag is accepted and the command runs successfully.
+/// This test confirms the CLI argument is properly wired up.
+#[tokio::test(flavor = "multi_thread", worker_threads = 1)]
+async fn accepts_add_dir_flag() -> anyhow::Result<()> {
+    let test = test_codex_exec();
+
+    let server = responses::start_mock_server().await;
+    let body = responses::sse(vec![
+        responses::ev_response_created("response_1"),
+        responses::ev_assistant_message("response_1", "Task completed"),
+        responses::ev_completed("response_1"),
+    ]);
+    responses::mount_sse_once(&server, body).await;
+
+    // Create temporary directories to use with --add-dir
+    let temp_dir1 = tempfile::tempdir()?;
+    let temp_dir2 = tempfile::tempdir()?;
+
+    test.cmd_with_server(&server)
+        .arg("--skip-git-repo-check")
+        .arg("--sandbox")
+        .arg("workspace-write")
+        .arg("--add-dir")
+        .arg(temp_dir1.path())
+        .arg("--add-dir")
+        .arg(temp_dir2.path())
+        .arg("test with additional directories")
+        .assert()
+        .code(0);
+
+    Ok(())
+}
+
+/// Verify that multiple --add-dir flags can be specified.
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn accepts_multiple_add_dir_flags() -> anyhow::Result<()> {
+    let test = test_codex_exec();
+
+    let server = responses::start_mock_server().await;
+    let body = responses::sse(vec![
+        responses::ev_response_created("response_1"),
+        responses::ev_assistant_message("response_1", "Multiple directories accepted"),
+        responses::ev_completed("response_1"),
+    ]);
+    responses::mount_sse_once(&server, body).await;
+
+    let temp_dir1 = tempfile::tempdir()?;
+    let temp_dir2 = tempfile::tempdir()?;
+    let temp_dir3 = tempfile::tempdir()?;
+
+    test.cmd_with_server(&server)
+        .arg("--skip-git-repo-check")
+        .arg("--sandbox")
+        .arg("workspace-write")
+        .arg("--add-dir")
+        .arg(temp_dir1.path())
+        .arg("--add-dir")
+        .arg(temp_dir2.path())
+        .arg("--add-dir")
+        .arg(temp_dir3.path())
+        .arg("test with three directories")
+        .assert()
+        .code(0);
+
+    Ok(())
+}