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_provision_git_user.py

@@ -64,11 +64,7 @@ def _plan(*, git_user: dict | None = None,  # type: ignore
         spec=spec,
         stage_dir=stage_dir or Path("/tmp/stage"),
         slug="demo-abc12",
-        container_name="bot-bottle-demo-abc12",
-        image="bot-bottle-claude:latest",
-        dockerfile_path="",
         forwarded_env={},
-        prompt_file=Path("/tmp/prompt.txt"),
         git_gate_plan=GitGatePlan(
             slug="demo-abc12",
             entrypoint_script=Path("/tmp/git-gate-entrypoint.sh"),
@@ -91,6 +87,8 @@ def _plan(*, git_user: dict | None = None,  # type: ignore
             image="bot-bottle-claude:latest",
             dockerfile="",
             guest_home="/home/node",
+            instance_name="bot-bottle-demo-abc12",
+            prompt_file=Path("/tmp/prompt.txt"),
             guest_env={},
         ),
     )