feat(orchestrator): slice 13c(iv) — gateway joins a shared network

The consolidated gateway ran on the default bridge; the model needs it on a
dedicated user-defined network that every agent bottle also joins, reaching
the gateway's egress / git-http / supervise ports by its address (no host
port publishing) — and the source IP the gateway attributes by is the
bottle's address on this network.

- GATEWAY_NETWORK = "bot-bottle-gateway"; DockerGateway gains a `network`
  param.
- ensure_running now ensures the network exists (idempotent create,
  tolerating a concurrent 'already exists') then runs with `--network`.
- Docker picks the subnet; the launcher reads it back (13c source-IP
  allocator) to hand each bottle a pinned address.

pyright 0 errors; pylint 9.83/10; unit suite green (the 13 test_sidecar_init
/bin/sleep errors are pre-existing NixOS-local noise).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WBMWTEtQdJ4W5UrWuLHCck

tests/unit/test_orchestrator_gateway.py

@@ -51,6 +51,33 @@ class TestDockerGateway(unittest.TestCase):
         self.assertEqual(1, len(runs))
         self.assertIn(self.sc.name, runs[0])
         self.assertIn("bot-bottle-sidecars:latest", runs[0])
+        # Runs on the shared gateway network so agents can reach it by IP.
+        self.assertEqual(self.sc.network, runs[0][runs[0].index("--network") + 1])
+
+    def test_ensure_running_creates_network_when_missing(self) -> None:
+        calls: list[list[str]] = []
+
+        def fake(argv: list[str]) -> Mock:
+            calls.append(argv)
+            if argv[:3] == ["docker", "network", "inspect"]:
+                return _proc(returncode=1, stderr="No such network")
+            return _proc(stdout="") if argv[:2] == ["docker", "ps"] else _proc()
+
+        with patch(_RUN_DOCKER, side_effect=fake):
+            self.sc.ensure_running()
+        creates = [c for c in calls if c[:3] == ["docker", "network", "create"]]
+        self.assertEqual([["docker", "network", "create", self.sc.network]], creates)
+
+    def test_ensure_running_reuses_existing_network(self) -> None:
+        calls: list[list[str]] = []
+
+        def fake(argv: list[str]) -> Mock:
+            calls.append(argv)
+            return _proc(stdout="") if argv[:2] == ["docker", "ps"] else _proc()
+
+        with patch(_RUN_DOCKER, side_effect=fake):
+            self.sc.ensure_running()  # network inspect returns 0 → exists
+        self.assertEqual([], [c for c in calls if c[:3] == ["docker", "network", "create"]])
 
     def test_ensure_running_raises_on_docker_failure(self) -> None:
         def fake(argv: list[str]) -> Mock: