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/cli.rs

@@ -52,6 +52,10 @@ pub struct Cli {
     #[arg(long = "skip-git-repo-check", default_value_t = false)]
     pub skip_git_repo_check: bool,
 
+    /// Additional directories that should be writable alongside the primary workspace.
+    #[arg(long = "add-dir", value_name = "DIR", value_hint = clap::ValueHint::DirPath)]
+    pub add_dir: Vec<PathBuf>,
+
     /// Path to a JSON Schema file describing the model's final response shape.
     #[arg(long = "output-schema", value_name = "FILE")]
     pub output_schema: Option<PathBuf>,