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,

tests/unit/test_contrib_claude_provider.py

@@ -78,6 +78,7 @@ def _plan(
             current_config_dir=Path("/tmp/current-config"),
         )
     return DockerBottlePlan(
+        guest_home="/home/node",
         spec=spec,
         stage_dir=Path("/tmp/stage"),
         slug="demo-abc12",

tests/unit/test_contrib_codex_provider.py

@@ -78,6 +78,7 @@ def _plan(
             current_config_dir=Path("/tmp/current-config"),
         )
     return DockerBottlePlan(
+        guest_home="/home/node",
         spec=spec,
         stage_dir=Path("/tmp/stage"),
         slug="demo-abc12",

tests/unit/test_docker_launch_teardown.py

@@ -44,6 +44,7 @@ def _plan(tmp: str) -> DockerBottlePlan:
         identity="test-teardown-00001",
     )
     return DockerBottlePlan(
+        guest_home="/home/node",
         spec=spec,
         stage_dir=stage,
         git_gate_plan=GitGatePlan(

tests/unit/test_docker_provision_git_user.py

@@ -40,6 +40,7 @@ def _plan(*, git_user: dict | None = None,
         copy_cwd=copy_cwd, user_cwd=user_cwd,
     )
     return DockerBottlePlan(
+        guest_home="/home/node",
         spec=spec,
         stage_dir=stage_dir or Path("/tmp/stage"),
         slug="demo-abc12",

tests/unit/test_plan_print_parity.py

@@ -103,6 +103,7 @@ def _proxy_plan(tmp: str) -> PipelockProxyPlan:
 def _docker_plan(spec: BottleSpec, tmp: str) -> DockerBottlePlan:
     stage = Path(tmp)
     return DockerBottlePlan(
+        guest_home="/home/node",
         spec=spec,
         stage_dir=stage,
         git_gate_plan=_git_gate_plan(tmp),
@@ -128,6 +129,7 @@ def _docker_plan(spec: BottleSpec, tmp: str) -> DockerBottlePlan:
 def _smolmachines_plan(spec: BottleSpec, tmp: str) -> SmolmachinesBottlePlan:
     stage = Path(tmp)
     return SmolmachinesBottlePlan(
+        guest_home="/home/node",
         spec=spec,
         stage_dir=stage,
         git_gate_plan=_git_gate_plan(tmp),

tests/unit/test_smolmachines_provision.py

@@ -120,6 +120,7 @@ def _plan(
             current_config_dir=Path("/tmp/current-config"),
         )
     return SmolmachinesBottlePlan(
+        guest_home="/home/node",
         spec=spec,
         stage_dir=stage_dir or Path("/tmp/stage"),
         slug="demo-abc12",