refactor: move guest_home onto AgentProvisionPlan as source of truth

guest_home is now a field on AgentProvisionPlan (set by each provider's
provision_plan() method). BottlePlan.guest_home becomes a read-only
property delegating to agent_provision.guest_home so existing callers
(provision_git, provision_skills, provision_prompt) are unchanged.

Both resolve_plan.py files drop guest_home from the plan constructor
call; the local variable still exists as an intermediary for the
workspace_plan call that precedes agent_provision_plan.

bot_bottle/backend/docker/resolve_plan.py

@@ -169,7 +169,6 @@ def resolve_plan(
     return DockerBottlePlan(
         spec=spec,
         stage_dir=stage_dir,
-        guest_home=guest_home,
         slug=slug,
         container_name=container_name,
         container_name_pinned=container_name_pinned,