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/agent_provider.py

@@ -103,6 +103,7 @@ class AgentProvisionPlan:
     prompt_mode: PromptMode
     image: str
     dockerfile: str
+    guest_home: str
     guest_env: dict[str, str]
     env_vars: dict[str, str] = field(default_factory=dict)
     dirs: tuple[AgentProvisionDir, ...] = ()