refactor(orchestrator): split conflated sidecar image into orchestrator + gateway images

One image — `bot-bottle-sidecars:latest`, built from `Dockerfile.sidecars` —
served two unrelated roles: the egress/git-gate/supervise *data plane* and
the orchestrator *control plane* (which ran the same image with the entrypoint
overridden to `python3 -m bot_bottle.orchestrator`). The control plane is
stdlib-only, so it needed none of the mitmproxy/git/gitleaks payload it was
riding on — while being the most secret-dense process on the host (PRD 0070's
"secret concentration").

Split into two purpose-built images:

- `Dockerfile.gateway` -> `bot-bottle-gateway:latest` — the data plane
  (renamed from Dockerfile.sidecars; identical contents).
- `Dockerfile.orchestrator` -> `bot-bottle-orchestrator:latest` — a lean
  `python:3.12-slim` runtime; the bind-mounted `bot_bottle` package supplies
  the code (so the #381 source-hash recreate semantics are unchanged).

`OrchestratorService` now takes distinct `image` (control plane, default
`ORCHESTRATOR_IMAGE`) and `gateway_image` (data plane, default `GATEWAY_IMAGE`)
instead of feeding one `self.image` to both, and builds the lean image
(build-if-missing) before starting the container. The per-bottle bundle
constants in `backend/docker/sidecar_bundle.py` now alias the gateway
constants so a bundle and the shared gateway can never drift onto different
images. The `bot-bottle-sidecars` *image* name and `Dockerfile.sidecars` are
gone; the per-bottle *container* name prefix (`bot-bottle-sidecars-<slug>`) is
intentionally left for a separate change.

Verified end-to-end: both images build; the lean image runs the control plane;
`ensure_running` brings up the orchestrator on `bot-bottle-orchestrator:latest`
and the gateway on `bot-bottle-gateway:latest` (distinct images) and reports
healthy.

Closes #384.

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

tests/README.md

@@ -48,7 +48,7 @@ Discovery is invoked with `-t .` (top-level dir = repo root) so the
   the bottle's runtime, and creates zero Docker resources.
 - `test_orphan_cleanup.py` — `network_remove` is idempotent against
   missing resources, so the EXIT trap can call it unconditionally.
-- `test_sidecar_bundle_image.py` — builds Dockerfile.sidecars and
+- `test_sidecar_bundle_image.py` — builds Dockerfile.gateway and
   probes that gitleaks / mitmdump / supervise are all reachable
   inside the bundle.
 - `test_sidecar_bundle_compose.py` — end-to-end compose-up of an