refactor(orchestrator): rename Sidecar -> Gateway for the consolidated data plane

Retire "sidecar" for the consolidated per-host path (PRD 0070 naming
decision): the orchestrator is the umbrella/control plane, and the
egress/git/supervise data-plane unit it runs is the "gateway".

- git mv sidecar.py -> gateway.py and the two integration + one unit test
  files; DockerSidecar->DockerGateway, Sidecar->Gateway,
  SidecarError->GatewayError, SIDECAR_*->GATEWAY_*, ensure_sidecar->
  ensure_gateway, sidecar_status->gateway_status, container name
  bot-bottle-orch-sidecar->bot-bottle-orch-gateway.
- Prose rename across broker/registry/egress/policy_resolver + PRD 0070.
- Preserved: the image name bot-bottle-sidecars, the
  BOT_BOTTLE_SIDECAR_IMAGE env var, Dockerfile.sidecars, and PRD 0069's
  own stage-name cross-references (that doc still uses "sidecar").

No behavior change. Full unit suite green (1679 tests; 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/integration/test_orchestrator_docker_gateway_build.py

@@ -0,0 +1,36 @@
+"""Integration: DockerGateway.image_exists reflects real docker state.
+
+Gated on a reachable Docker daemon. Deliberately does NOT build the full
+sidecar bundle (a heavy, slow image build) — it exercises the `image_exists`
+primitive that `ensure_built` gates on, against a tiny pulled image and a
+name that can't exist.
+"""
+
+from __future__ import annotations
+
+import subprocess
+import unittest
+
+from bot_bottle.orchestrator.gateway import DockerGateway
+from tests._docker import skip_unless_docker
+
+IMAGE = "busybox"
+
+
+@skip_unless_docker()
+class TestDockerGatewayImageExists(unittest.TestCase):
+    def test_image_exists_true_for_present_false_for_absent(self) -> None:
+        # Ensure the tiny image is present (build_if_missing is disabled here
+        # so this never triggers a Dockerfile.sidecars build).
+        subprocess.run(
+            ["docker", "pull", IMAGE],
+            stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL, check=False,
+        )
+        self.assertTrue(DockerGateway(IMAGE, dockerfile=None).image_exists())
+        self.assertFalse(
+            DockerGateway("bot-bottle-nonexistent:doesnotexist", dockerfile=None).image_exists()
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()