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/git_http_backend.py

@@ -28,7 +28,7 @@ from pathlib import Path
 from urllib.parse import urlsplit
 
 # policy_resolver ships flat alongside this file in the sidecar bundle
-# image (see Dockerfile.sidecars); the bot_bottle.* fallback is the
+# image (see Dockerfile.gateway); the bot_bottle.* fallback is the
 # host-side / test path. Mirrors egress_addon's import shape.
 try:
     from policy_resolver import (  # type: ignore[import-not-found]
@@ -104,7 +104,7 @@ def resolve_sandbox_root(
 
 # Mirrors git_gate_render.GIT_GATE_TIMEOUT_SECS. Duplicated rather than
 # imported: this module ships as a flat top-level sibling in the sidecar
-# bundle image (see Dockerfile.sidecars), not as part of the bot_bottle
+# bundle image (see Dockerfile.gateway), not as part of the bot_bottle
 # package, so `bot_bottle.git_gate` and its dependency chain aren't
 # available at runtime.
 GIT_GATE_TIMEOUT_SECS = 15