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/__init__.py

@@ -78,9 +78,12 @@ class BottlePlan(ABC):
 
     spec: BottleSpec
     stage_dir: Path
-    guest_home: str
     git_gate_plan: GitGatePlan
 
+    @property
+    def guest_home(self) -> str:
+        return self.agent_provision.guest_home
+
     @property
     def git_gate_insteadof_host(self) -> str:
         """Host (and optional port) used in git-gate insteadOf URLs.