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/src/lib.rs

@@ -62,6 +62,7 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         dangerously_bypass_approvals_and_sandbox,
         cwd,
         skip_git_repo_check,
+        add_dir,
         color,
         last_message_file,
         json: json_mode,
@@ -180,7 +181,7 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         show_raw_agent_reasoning: oss.then_some(true),
         tools_web_search_request: None,
         experimental_sandbox_command_assessment: None,
-        additional_writable_roots: Vec::new(),
+        additional_writable_roots: add_dir,
     };
     // Parse `-c` overrides.
     let cli_kv_overrides = match config_overrides.parse_overrides() {