refactor(backend): hoist guest_home to BottlePlan base

Per PR review feedback (review #132): guest_home shouldn't be
buried inside workspace_plan / read from a hardcoded literal in
each provision module. It's a cross-cutting bottle property — the
backend's prepare step knows it, and every downstream consumer
(contrib providers, git provisioning, gitconfig path) should
read it from one place.

- Adds guest_home: str to BottlePlan base dataclass.
- Both backends' prepare steps populate plan.guest_home.
- contrib/{claude,codex}/agent_provider.py read plan.guest_home
  (was plan.workspace_plan.guest_home).
- bot_bottle/backend/docker/provision/git.py reads plan.guest_home
  for the gitconfig destination (was hardcoded "/home/node").
- bot_bottle/backend/smolmachines/provision/git.py drops the
  _GUEST_HOME / _guest_home() helpers and reads plan.guest_home.
- Tests that construct BottlePlan subclasses directly pass
  guest_home="/home/node" explicitly.

bot_bottle/backend/__init__.py

@@ -76,6 +76,7 @@ class BottlePlan(ABC):
 
     spec: BottleSpec
     stage_dir: Path
+    guest_home: str
     git_gate_plan: GitGatePlan
     egress_plan: EgressPlan
     supervise_plan: SupervisePlan | None