refactor(agent_provider): drop GUEST_HOME default, backend drives guest_home

Per PR review feedback (review #130): the GUEST_HOME = '/home/node'
default in agent_provider.py was driving the wrong direction —
the agent provider shouldn't ship its own opinion about the guest
home, the backend should.

- Removes the GUEST_HOME constant.
- Makes guest_home a required kwarg on AgentProvider.provision_plan
  and the agent_provision_plan shim (no default).
- Drops module-level _SKILLS_DIR / _PROMPT_PATH constants from
  contrib/{claude,codex}/agent_provider.py; both providers now
  derive the in-guest paths from plan.workspace_plan.guest_home
  at call time, which the backend's prepare step populated.
- Updates tests/unit/test_agent_provider.py callers to pass
  guest_home explicitly. The backend prepare paths already pass
  it; no production-code call sites changed.

bot_bottle/agent_provider.py

@@ -41,8 +41,6 @@ PROVIDER_TEMPLATES = frozenset({PROVIDER_CLAUDE, PROVIDER_CODEX})
 CODEX_HOST_CREDENTIAL_HOSTS = ("api.openai.com", "chatgpt.com")
 PromptMode = Literal["append_file", "read_prompt_file"]
 
-GUEST_HOME = "/home/node"
-
 
 @dataclass(frozen=True)
 class AgentProviderRuntime:
@@ -131,7 +129,7 @@ class AgentProvider(ABC):
         *,
         dockerfile: str,
         state_dir: Path,
-        guest_home: str = GUEST_HOME,
+        guest_home: str,
         guest_env: dict[str, str] | None = None,
         auth_token: str = "",
         forward_host_credentials: bool = False,
@@ -201,7 +199,7 @@ def agent_provision_plan(
     template: str,
     dockerfile: str,
     state_dir: Path,
-    guest_home: str = GUEST_HOME,
+    guest_home: str,
     guest_env: dict[str, str] | None = None,
     auth_token: str = "",
     forward_host_credentials: bool = False,