New decnet/config_ini.py parses a role-scoped INI file via stdlib
configparser and seeds os.environ via setdefault — real env vars still
win, keeping full back-compat with .env.local flows.
[decnet] holds role-agnostic keys (mode, disallow-master, log-file-path);
the role section matching `mode` is loaded, the other is ignored
silently so a worker never reads master-only keys (and vice versa).
Loader is standalone in this commit — not wired into cli.py yet.
