File tools (read_file, write_file, patch, list_directory, etc.) used
os.path.expanduser() which reads the gateway process HOME env var.
In Docker/systemd/s6 deployments where the gateway HOME differs from
interactive sessions, tilde expanded to the wrong directory.
Add _expand_tilde() helper that delegates to get_subprocess_home() when
available, falling back to os.path.expanduser(). Replace all 9
expanduser() call sites in file_tools.py with _expand_tilde().
