[POC] Add native environment io tools

[lt-oai]

Why

Ground the environment file I/O proposal in the existing Codex environment/filesystem code path instead of introducing a separate MCP server per environment.

What

• Add an experimental native io namespace gated by model experimental_supported_tools: ["io"].
• Register filesystem-MCP-shaped P0 tools: read_file, write_file, edit_file, create_directory, list_directory, get_file_info, and list_allowed_directories.
• Route calls through the selected turn environment ExecutorFileSystem, so local environments use LocalFileSystem and remote environments use RemoteFileSystem / exec-server fs/* methods.
• Add a remote routing test that writes different local/remote files and verifies io.read_file reads from the selected remote environment.

Testing

• cargo fmt
• cargo test -p codex-core io_namespace_registers_for_experimental_supported_tool
• cargo test -p codex-core io_read_file_routes_to_selected_remote_environment