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_plan_print_parity.py

@@ -79,14 +79,16 @@ def _egress_plan(tmp: str) -> EgressPlan:
     )
 
 
-def _agent_provision() -> AgentProvisionPlan:
+def _agent_provision(tmp: str) -> AgentProvisionPlan:
     return AgentProvisionPlan(
         template="claude",
         command="claude",
         prompt_mode="append_file",
-        image="",
+        image="bot-bottle-claude:latest",
         dockerfile="",
         guest_home="/home/node",
+        instance_name="bot-bottle-test-00001",
+        prompt_file=Path(tmp) / "prompt.txt",
         guest_env={"HTTPS_PROXY": "http://127.0.0.1:9999"},
     )
 
@@ -99,13 +101,9 @@ def _docker_plan(spec: BottleSpec, tmp: str) -> DockerBottlePlan:
         git_gate_plan=_git_gate_plan(tmp),
         egress_plan=_egress_plan(tmp),
         supervise_plan=None,
-        agent_provision=_agent_provision(),
+        agent_provision=_agent_provision(tmp),
         slug="test-00001",
-        container_name="bot-bottle-test-00001",
-        image="bot-bottle-claude:latest",
-        dockerfile_path="",
         forwarded_env={},
-        prompt_file=stage / "prompt.txt",
         use_runsc=False,
     )
 
@@ -118,15 +116,12 @@ def _smolmachines_plan(spec: BottleSpec, tmp: str) -> SmolmachinesBottlePlan:
         git_gate_plan=_git_gate_plan(tmp),
         egress_plan=_egress_plan(tmp),
         supervise_plan=None,
-        agent_provision=_agent_provision(),
+        agent_provision=_agent_provision(tmp),
         slug="test-00001",
         bundle_subnet="10.99.0.0/24",
         bundle_gateway="10.99.0.1",
         bundle_ip="10.99.0.2",
-        machine_name="bot-bottle-test-00001",
-        agent_image="bot-bottle-claude:latest",
         guest_env={"HTTPS_PROXY": "http://127.0.0.1:9999"},
-        prompt_file=stage / "prompt.txt",
     )