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/contrib/codex/agent_provider.py

@@ -67,7 +67,8 @@ class CodexAgentProvider(AgentProvider):
         *,
         dockerfile: str,
         state_dir: Path,
-        guest_home: str,
+        instance_name: str,
+        prompt_file: Path,
         guest_env: dict[str, str] | None = None,
         auth_token: str = "",
         forward_host_credentials: bool = False,
@@ -78,6 +79,7 @@ class CodexAgentProvider(AgentProvider):
     ) -> AgentProvisionPlan:
         del auth_token, label, color  # Claude-only knobs
         resolved_guest_env = dict(guest_env or {})
+        guest_home = self.guest_home
         trusted_path = trusted_project_path or guest_home
 
         env_vars: dict[str, str] = {
@@ -148,6 +150,8 @@ class CodexAgentProvider(AgentProvider):
             image=_RUNTIME.image,
             dockerfile=dockerfile,
             guest_home=guest_home,
+            instance_name=instance_name,
+            prompt_file=prompt_file,
             env_vars=env_vars,
             guest_env=resolved_guest_env,
             dirs=tuple(dirs),