feat(env): run decnet.ini loader at package import; expose DECNET_MODE

- 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.

decnet/__init__.py

@@ -0,0 +1,12 @@
+"""DECNET — honeypot deception-network framework.
+
+This __init__ runs once, on the first `import decnet.*`. It seeds
+os.environ from /etc/decnet/decnet.ini (if present) so that later
+module-level reads in decnet.env pick up the INI values as if they had
+been exported by the shell. Real env vars always win via setdefault().
+
+Kept minimal on purpose — any heavier work belongs in a submodule.
+"""
+from decnet.config_ini import load_ini_config as _load_ini_config
+
+_load_ini_config()