- decnet/__init__.py now calls load_ini_config() on first import of any
decnet.* module, seeding os.environ via setdefault() so env.py's
module-level reads pick up INI values before the shell had to export
them. Real env vars still win.
- env.py exposes DECNET_MODE (default 'master') and
DECNET_DISALLOW_MASTER (default true), consumed by the upcoming
master-command gating in cli.py.
Back-compat: missing /etc/decnet/decnet.ini is a no-op. Existing
.env.local + flag-based launches behave identically.