feat(cli): add topology sub-command group

decnet topology {generate,list,show,deploy,teardown} wraps the new
persistence and deployer APIs. Structured text output, no ASCII art —
visual DAG rendering belongs in the web dashboard. Group is master-only
via MASTER_ONLY_GROUPS and a _require_master_mode guard on each body.

decnet/cli/__init__.py

@@ -31,6 +31,7 @@ from . import (
     sniffer,
     swarm,
     swarmctl,
+    topology,
     updater,
     web,
     workers,
@@ -50,6 +51,7 @@ for _mod in (
     swarm,
     deploy, lifecycle, workers, inventory,
     web, profiler, sniffer, db,
+    topology,
 ):
     _mod.register(app)