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/mod.rs

@@ -1,4 +1,5 @@
 // Aggregates all former standalone integration tests as modules.
+mod add_dir;
 mod apply_patch;
 mod auth_env;
 mod originator;