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/docker/provision/git.py

@@ -57,7 +57,7 @@ def _provision_git_gate_config(plan: DockerBottlePlan, bottle: Bottle) -> None:
     manifest_bottle = plan.spec.manifest.bottle_for(plan.spec.agent_name)
     if not manifest_bottle.git:
         return
-    container_gitconfig = "/home/node/.gitconfig"
+    container_gitconfig = f"{plan.guest_home}/.gitconfig"
 
     content = git_gate_render_gitconfig(manifest_bottle.git, GIT_GATE_HOSTNAME)
     config_file = plan.stage_dir / "agent_gitconfig"