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/contrib/codex/agent_provider.py

@@ -168,7 +168,7 @@ class CodexAgentProvider(AgentProvider):
         agent = plan.spec.manifest.agents[plan.spec.agent_name]
         if not agent.skills:
             return
-        skills_dir = _skills_dir(plan.workspace_plan.guest_home)
+        skills_dir = _skills_dir(plan.guest_home)
         bottle.exec(f"mkdir -p {skills_dir}", user="root")
         for name in agent.skills:
             src = host_skill_dir(name)
@@ -188,7 +188,7 @@ class CodexAgentProvider(AgentProvider):
         Codex reads it via the agent's `Read and follow the
         instructions in <path>.` bootstrap (see `prompt_args`); the
         file is copied either way so the path always exists."""
-        prompt_path = _prompt_path(plan.workspace_plan.guest_home)
+        prompt_path = _prompt_path(plan.guest_home)
         bottle.cp_in(str(plan.prompt_file), prompt_path)
         bottle.exec(
             f"chown node:node {prompt_path} && chmod 600 {prompt_path}",