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.

tests/unit/test_compose.py

@@ -164,6 +164,7 @@ def _plan(
 
     spec = _spec(supervise=supervise, with_git=with_git, with_egress=with_egress)
     return DockerBottlePlan(
+        guest_home="/home/node",
         spec=spec,
         stage_dir=STAGE,
         slug=SLUG,