refactor(firecracker): orchestrator under the Orchestrator service ABC

Pull the control plane's host-side logic out of `infra_vm` into
`FirecrackerOrchestrator` (backend/firecracker/orchestrator.py): booting the
orchestrator microVM on its link with the persistent registry volume (/dev/vdb),
seeding the host-canonical signing key over SSH, waiting for /health, and the
buildah SSH target. `url()` == `gateway_url()` (the gateway resolves the
orchestrator by guest IP via bb_orch). This completes the trio — docker, macOS,
firecracker — behind both the Gateway and Orchestrator ABCs.

`infra_vm` stays the pair coordinator + shared substrate: `ensure_running` now
mints nothing itself — it constructs both services (lazy import to break the
cycle), calls `orchestrator.ensure_running()` first, then
`gateway.connect_to_orchestrator(orch.gateway_url(), orch.mint_gateway_token())`.
The plane-agnostic boot primitives graduate to public API (`_boot_vm`/
`_push_secret` -> `boot_vm`/`push_secret`) now that they're consumed only across
modules; `InfraVm` loses its `orchestrator_url` (moved to the service) and
`InfraEndpoint.orchestrator` is now the `FirecrackerOrchestrator` service.

Container-lifecycle tests split into test_firecracker_orchestrator; the infra_vm
tests keep the substrate + pair-coordinator coverage.

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

bot_bottle/backend/firecracker/image_builder.py

@@ -126,7 +126,7 @@ def _build_in_infra(
     and stream its rootfs into `base`. The infra VMs persist; only the
     per-build container/image/context are cleaned up."""
     orchestrator = infra_vm.ensure_running().orchestrator
-    key, ip = orchestrator.private_key, orchestrator.guest_ip
+    key, ip = orchestrator.ssh_target()
     tag = f"bot-bottle-agent-build-{digest}"
     ctx = f"/tmp/agent-build-{digest}"
     smoke_ctr, export_ctr = f"{tag}-smoke", f"{tag}-export"