feat(enroll): systemd units for agent/forwarder/engine + log-directory INI key

Rename log-file-path -> log-directory (maps to DECNET_LOG_DIRECTORY). Bundle
now ships three systemd units rendered with agent_name/master_host and installs
them into /etc/systemd/system/. Bootstrap replaces direct 'decnet X --daemon'
calls with systemctl enable --now. Each unit pins DECNET_SYSTEM_LOGS so agent,
forwarder, and deckies logs land at decnet.{agent,forwarder}.log and decnet.log
under /var/log/decnet.

tests/test_config_ini.py

@@ -95,17 +95,17 @@ agent-port = 8765
 
 
 def test_common_keys_always_exported(monkeypatch, tmp_path):
-    _scrub(monkeypatch, "DECNET_MODE", "DECNET_DISALLOW_MASTER", "DECNET_LOG_FILE_PATH")
+    _scrub(monkeypatch, "DECNET_MODE", "DECNET_DISALLOW_MASTER", "DECNET_LOG_DIRECTORY")
     ini = _write_ini(tmp_path, """
 [decnet]
 mode = agent
 disallow-master = true
-log-file-path = /var/log/decnet/decnet.log
+log-directory = /var/log/decnet
 """)
     load_ini_config(ini)
     assert os.environ["DECNET_MODE"] == "agent"
     assert os.environ["DECNET_DISALLOW_MASTER"] == "true"
-    assert os.environ["DECNET_LOG_FILE_PATH"] == "/var/log/decnet/decnet.log"
+    assert os.environ["DECNET_LOG_DIRECTORY"] == "/var/log/decnet"
 
 
 def test_invalid_mode_raises(monkeypatch, tmp_path):