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

bot_bottle/egress_addon_core.py

@@ -8,7 +8,7 @@ container.
 
 Imports: stdlib + `yaml_subset` (which is itself stdlib-only and
 ships flat into the sidecar bundle image alongside this file —
-see `Dockerfile.sidecars`)."""
+see `Dockerfile.gateway`)."""
 
 from __future__ import annotations
 
@@ -22,7 +22,7 @@ except ImportError:  # pragma: no cover - host-side path
     from .yaml_subset import YamlSubsetError, parse_yaml_subset
 
 # DLP detector-config parsing lives in a sibling module (also flat-bundled
-# into the gateway — see Dockerfile.sidecars). Re-exported below so existing
+# into the gateway — see Dockerfile.gateway). Re-exported below so existing
 # `from egress_addon_core import ON_MATCH_*` callers keep working.
 try:
     from egress_dlp_config import (  # type: ignore[import-not-found]