refactor: remove leftovers from the orchestrator/gateway consolidation

Sweep for vestiges of the old combined-plane model and the pre-split shared
rootfs. Two are load-bearing, the rest are stale docs/comments:

- Bug: macOS `enumerate_active` only excluded the gateway container from the
  agent list, so after the split the orchestrator container
  (`bot-bottle-mac-orchestrator`, also `bot-bottle-`-prefixed) was enumerated as
  a phantom agent. Exclude both infra containers; test covers it.
- Dead code: the gateway `bootstrap.py` still carried an `orchestrator` daemon
  spec + `_OPT_IN_DAEMONS` + a signing-key/JWT env branch, all for the old
  combined container where the gateway process could also run the control plane.
  No backend ever requests it now — removed; the key-stripping stays as
  defense-in-depth.

Stale-comment reframes: "the/single infra container" -> the orchestrator +
gateway pair (or the specific plane); "shared rootfs / bb_role init / one
published rootfs" -> the per-plane rootfs + `role_init`; the deleted
Dockerfile.infra references in Dockerfile.orchestrator/.gateway; and the macOS
"one infra container ... same address" docstring + its now-false
share-one-address test (the planes are distinct containers with distinct
addresses).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/backend/docker/__init__.py

@@ -11,7 +11,7 @@ The bulk of the implementation lives in sibling modules:
   - launch:               bring-up + teardown context manager
   - cleanup:              orphan enumeration, removal, active listing
   - backend:              DockerBottleBackend façade wiring the above
-  - infra:                DockerInfraService (the per-host infra container)
+  - infra:                DockerInfraService (the per-host orchestrator + gateway pair)
 
 Thin by design: the public names are re-exported lazily via `__getattr__`, so
 importing a leaf like `backend.docker.util` doesn't drag `DockerBottleBackend`