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/docker/backend.py

@@ -105,6 +105,10 @@ class DockerBottleBackend(BottleBackend["DockerBottlePlan", "DockerBottleCleanup
         with _launch.launch(plan, provision=self.provision) as bottle:
             yield bottle
 
+    def ensure_orchestrator(self) -> str:
+        from ...orchestrator.lifecycle import OrchestratorService
+        return OrchestratorService().ensure_running()
+
     def supervise_mcp_url(self, plan: DockerBottlePlan) -> str:
         """Docker bottles reach the supervise daemon via the
         compose-network alias `supervise:9100`. No per-bottle URL