feat(supervise): start orchestrator on demand via backend-agnostic bring-up

Add BottleBackend.ensure_orchestrator() -> str: the backend-agnostic
entry point that brings up the per-host orchestrator + shared gateway
(idempotent) and returns its host-reachable control-plane URL. Docker
starts the orchestrator + gateway containers (OrchestratorService);
firecracker boots the infra VM; macos-container dies with a pointer
(no orchestrator). Previously bring-up was reachable only through each
backend's consolidated_launch, with no shared handle.

Wire it into `bot-bottle supervise`: supervise is often the first thing
an operator runs, before any bottle has booted the control plane, so
`_resolve_orchestrator_url` now starts the selected backend's
orchestrator on demand when discovery finds nothing, instead of failing
with "launch a bottle first".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UoEZHDjv84ChoZbozQERhJ

bot_bottle/backend/firecracker/backend.py

@@ -110,3 +110,7 @@ class FirecrackerBottleBackend(
 
     def supervise_mcp_url(self, plan: FirecrackerBottlePlan) -> str:
         return plan.agent_supervise_url
+
+    def ensure_orchestrator(self) -> str:
+        from . import infra_vm
+        return infra_vm.ensure_running().control_plane_url