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/smolmachines/resolve_plan.py

@@ -51,10 +51,6 @@ def resolve_plan(
     spec: BottleSpec,
     slug: str,
     resolved_env: ResolvedEnv,
-    instance_name: str,
-    agent_image: str,
-    agent_dockerfile_path: str,
-    prompt_file: Path,
     agent_provision_plan: AgentProvisionPlan,
     egress_plan: EgressPlan,
     supervise_plan: SupervisePlan,
@@ -79,10 +75,7 @@ def resolve_plan(
         bundle_subnet=subnet,
         bundle_gateway=gateway,
         bundle_ip=bundle_ip,
-        machine_name=instance_name,
-        agent_image=agent_image,
         guest_env=agent_provision_plan.guest_env,
-        prompt_file=prompt_file,
         git_gate_plan=git_gate_plan,
         egress_plan=egress_plan,
         supervise_plan=supervise_plan,