feat(bus): host-local UNIX-socket pub/sub worker (DEBT-029)

Land the `decnet bus` worker and `get_bus()` factory. Transport is a
host-local UNIX-domain socket (0660, group=decnet); authz is the file
mode. Wire framing is a tiny verb-line + 4-byte-BE length + orjson body.
NATS-style wildcard topics (`*`, `>`). At-most-once, fire-and-forget —
DB stays the source of truth. `FakeBus` / `NullBus` for tests and the
disabled path. Cross-host federation is deferred to a future
`--bridge-tcp` mode; DEBT-030 is master-only and unblocked.

decnet/cli/__init__.py

@@ -21,6 +21,7 @@ import typer
 from . import (
     agent,
     api,
+    bus,
     db,
     deploy,
     forwarder,
@@ -51,7 +52,7 @@ for _mod in (
     swarm,
     deploy, lifecycle, workers, inventory,
     web, profiler, sniffer, db,
-    topology,
+    topology, bus,
 ):
     _mod.register(app)