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/smolmachines/provision/git.py

@@ -36,14 +36,6 @@ from ... import Bottle
 from ..bottle_plan import SmolmachinesBottlePlan
 
 
-# `node` is the agent user from the repo Dockerfile.
-_GUEST_HOME = "/home/node"
-
-
-def _guest_home() -> str:
-    return _GUEST_HOME
-
-
 def provision_git(plan: SmolmachinesBottlePlan, bottle: Bottle) -> None:
     """Set up git inside the guest. Runs all three subcases; each
     no-ops when its condition isn't met."""
@@ -92,7 +84,7 @@ def _provision_git_gate_config(
         manifest_bottle.git, plan.agent_git_gate_host, scheme="http",
     )
 
-    guest_gitconfig = f"{_guest_home()}/.gitconfig"
+    guest_gitconfig = f"{plan.guest_home}/.gitconfig"
     # Stage the file under the plan's stage_dir so cp_in
     # has a stable host path. The plan's stage_dir is cleaned up
     # by start.py's session-end teardown.