refactor(box): remove legacy in-process runtime code and clean up smells

After the architecture settled on always using an independent Box Runtime
service, several pieces of compatibility code and design shortcuts were
left behind. This commit cleans them up:

- Remove `LocalBoxRuntimeClient` and `create_box_runtime_client` from
  production code (moved to test-only helper).
- Remove unused `_clip_bytes` method from backend.
- Remove `__langbot_session_placeholder__` hack by making `BoxSpec.cmd`
  default to empty and validating non-empty only in `runtime.execute()`.
- Extract `get_box_config()` helper to eliminate 5× duplicated config
  access boilerplate.
- Remove `session_id`/`host_path`/`host_path_mode` from the LLM-facing
  tool schema to enforce request-scoped session isolation.
- Fix dual shutdown path: `NativeToolLoader.shutdown()` no longer calls
  `box_service.shutdown()` (handled by `Application.dispose()`).
- Simplify `_assert_session_compatible` with a loop.
- Inline client creation in `BoxRuntimeConnector`.
- Remove redundant `BOX__RUNTIME_URL` env var from docker-compose
  (auto-detected by code).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/langbot/pkg/box/client.py

@@ -1,4 +1,4 @@
-"""BoxRuntimeClient abstraction for local and remote Box Runtime access."""
+"""BoxRuntimeClient abstraction for remote Box Runtime access."""
 
 from __future__ import annotations
 
@@ -16,8 +16,7 @@ from .errors import (
     BoxSessionNotFoundError,
     BoxValidationError,
 )
-from .models import BoxExecutionResult, BoxExecutionStatus, BoxSpec
-from .runtime import BoxRuntime
+from .models import BoxExecutionResult, BoxExecutionStatus, BoxSpec, get_box_config
 from ..utils import platform
 
 if TYPE_CHECKING:
@@ -34,9 +33,7 @@ _ERROR_CODE_MAP: dict[str, type[BoxError]] = {
 
 
 def resolve_box_runtime_url(ap: 'core_app.Application') -> str:
-    box_config = getattr(ap, 'instance_config', None)
-    box_config_data = getattr(box_config, 'data', {}) if box_config is not None else {}
-    runtime_url = str(box_config_data.get('box', {}).get('runtime_url', '')).strip()
+    runtime_url = str(get_box_config(ap).get('runtime_url', '')).strip()
     if runtime_url:
         return runtime_url
 
@@ -45,16 +42,6 @@ def resolve_box_runtime_url(ap: 'core_app.Application') -> str:
     return 'http://127.0.0.1:5410'
 
 
-def create_box_runtime_client(
-    ap: 'core_app.Application',
-    runtime_url: str | None = None,
-) -> 'RemoteBoxRuntimeClient':
-    return RemoteBoxRuntimeClient(
-        base_url=runtime_url or resolve_box_runtime_url(ap),
-        logger=ap.logger,
-    )
-
-
 class BoxRuntimeClient(abc.ABC):
     """Abstract interface that BoxService uses to talk to a Box Runtime."""
 
@@ -83,41 +70,6 @@ class BoxRuntimeClient(abc.ABC):
     async def create_session(self, spec: BoxSpec) -> dict: ...
 
 
-class LocalBoxRuntimeClient(BoxRuntimeClient):
-    """In-process client that wraps a real BoxRuntime directly."""
-
-    def __init__(self, logger: logging.Logger, runtime: BoxRuntime | None = None):
-        self._runtime = runtime or BoxRuntime(logger=logger)
-
-    @property
-    def runtime(self) -> BoxRuntime:
-        return self._runtime
-
-    async def initialize(self) -> None:
-        await self._runtime.initialize()
-
-    async def execute(self, spec: BoxSpec) -> BoxExecutionResult:
-        return await self._runtime.execute(spec)
-
-    async def shutdown(self) -> None:
-        await self._runtime.shutdown()
-
-    async def get_status(self) -> dict:
-        return await self._runtime.get_status()
-
-    async def get_sessions(self) -> list[dict]:
-        return self._runtime.get_sessions()
-
-    async def get_backend_info(self) -> dict:
-        return await self._runtime.get_backend_info()
-
-    async def delete_session(self, session_id: str) -> None:
-        await self._runtime.delete_session(session_id)
-
-    async def create_session(self, spec: BoxSpec) -> dict:
-        return await self._runtime.create_session(spec)
-
-
 class RemoteBoxRuntimeClient(BoxRuntimeClient):
     """HTTP client that talks to a standalone Box Runtime service."""