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)
 

decnet/cli/bus.py

@@ -0,0 +1,45 @@
+from __future__ import annotations
+
+import typer
+
+from . import utils as _utils
+from .utils import console, log
+
+
+def register(app: typer.Typer) -> None:
+    @app.command(name="bus")
+    def bus_cmd(
+        socket_path: str = typer.Option(
+            None, "--socket", "-s",
+            help="UNIX socket path (defaults to DECNET_BUS_SOCKET env var, "
+                 "then /run/decnet/bus.sock, then ~/.decnet/bus.sock).",
+        ),
+        group: str = typer.Option(
+            "decnet", "--group", "-g",
+            help="POSIX group to chown the socket to (falls back to process "
+                 "group if the named group does not exist).",
+        ),
+        heartbeat: int = typer.Option(
+            10, "--heartbeat", "-H",
+            help="Seconds between system.bus.health heartbeat events.",
+        ),
+        daemon: bool = typer.Option(False, "--daemon", "-d", help="Detach to background as a daemon process."),
+    ) -> None:
+        """Run the DECNET ServiceBus worker (host-local UNIX-socket pub/sub)."""
+        import asyncio
+        from decnet.bus.factory import _default_socket_path
+        from decnet.bus.worker import bus_worker
+
+        resolved = socket_path or _default_socket_path()
+
+        if daemon:
+            log.info("bus daemonizing socket=%s", resolved)
+            _utils._daemonize()
+
+        log.info("bus starting socket=%s group=%s heartbeat=%ds", resolved, group, heartbeat)
+        console.print(f"[bold cyan]Bus starting[/] (socket: {resolved}, heartbeat: {heartbeat}s)")
+
+        try:
+            asyncio.run(bus_worker(resolved, group=group, heartbeat_interval=heartbeat))
+        except KeyboardInterrupt:
+            console.print("\n[yellow]Bus stopped.[/]")