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.

tests/unit/test_docker_provision_git_user.py

@@ -61,7 +61,6 @@ def _plan(*, git_user: dict | None = None,  # type: ignore
         copy_cwd=copy_cwd, user_cwd=user_cwd,
     )
     return DockerBottlePlan(
-        guest_home="/home/node",
         spec=spec,
         stage_dir=stage_dir or Path("/tmp/stage"),
         slug="demo-abc12",
@@ -95,6 +94,7 @@ def _plan(*, git_user: dict | None = None,  # type: ignore
             prompt_mode="append_file",
             image="bot-bottle-claude:latest",
             dockerfile="",
+            guest_home="/home/node",
             guest_env={},
         ),
         workspace_plan=workspace_plan(spec, guest_home="/home/node"),