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>

tests/unit/test_docker_launch_teardown.py

@@ -63,17 +63,15 @@ def _plan(tmp: str) -> DockerBottlePlan:
             template="claude",
             command="claude",
             prompt_mode="append_file",
-            image="",
+            image="bot-bottle-claude:latest",
             dockerfile="",
             guest_home="/home/node",
+            instance_name="bot-bottle-test-teardown-abc",
+            prompt_file=stage / "prompt.txt",
             guest_env={},
         ),
         slug="test-teardown-00001",
-        container_name="bot-bottle-test-teardown-abc",
-        image="bot-bottle-claude:latest",
-        dockerfile_path="",
         forwarded_env={},
-        prompt_file=stage / "prompt.txt",
         use_runsc=False,
     )