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

bot_bottle/egress_addon.py

@@ -1,4 +1,4 @@
-"""mitmproxy addon entrypoint for the egress sidecar (PRD 0017, PRD 0053).
+"""mitmproxy addon entrypoint for the egress gateway (PRD 0017, PRD 0053).
 
 Loaded by `mitmdump -s /app/egress_addon.py` inside the
 egress container."""
@@ -275,7 +275,7 @@ class EgressAddon:
                 return
 
         # Strip agent-set Authorization after DLP scan so smuggled tokens
-        # are caught above; the route may inject sidecar-owned auth below.
+        # are caught above; the route may inject gateway-owned auth below.
         flow.request.headers.pop("authorization", None)
 
         # Build headers mapping for match evaluation