refactor: make AgentProvisionPlan the source of truth for instance_name, prompt_file, image, dockerfile, guest_home

Drop the parallel fields passed through prepare() → _resolve_plan and
read everything from agent_provision instead. The provider plugin now
declares its own guest_home (so the backend stops hardcoding
"/home/node") and the wrapper that builds the provision plan accepts
instance_name and prompt_file, which providers store on the plan.

DockerBottlePlan and SmolmachinesBottlePlan expose container_name /
machine_name, image / agent_image, dockerfile_path /
agent_dockerfile_path, and prompt_file as properties that delegate to
agent_provision so existing call sites keep working unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot_bottle/backend/docker/backend.py

@@ -54,11 +54,7 @@ class DockerBottleBackend(BottleBackend["DockerBottlePlan", "DockerBottleCleanup
         *,
         slug: str,
         resolved_env,
-        instance_name: str,
-        agent_image: str,
-        prompt_file: Path,
         agent_provision_plan,
-        agent_dockerfile_path: str,
         egress_plan,
         git_gate_plan,
         supervise_plan,
@@ -68,10 +64,6 @@ class DockerBottleBackend(BottleBackend["DockerBottlePlan", "DockerBottleCleanup
             spec,
             slug=slug,
             resolved_env=resolved_env,
-            instance_name=instance_name,
-            agent_image=agent_image,
-            agent_dockerfile_path=agent_dockerfile_path,
-            prompt_file=prompt_file,
             agent_provision_plan=agent_provision_plan,
             egress_plan=egress_plan,
             supervise_plan=supervise_plan,