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/connector.py

@@ -6,7 +6,8 @@ import sys
 from typing import TYPE_CHECKING
 
 from .errors import BoxRuntimeUnavailableError
-from .client import create_box_runtime_client, resolve_box_runtime_url
+from .client import RemoteBoxRuntimeClient, resolve_box_runtime_url
+from .models import get_box_config
 from ..utils import platform
 
 if TYPE_CHECKING:
@@ -24,7 +25,7 @@ class BoxRuntimeConnector:
         self.configured_runtime_url = self._load_configured_runtime_url()
         self.runtime_url = self.configured_runtime_url or resolve_box_runtime_url(ap)
         self.manages_local_runtime = self._should_manage_local_runtime()
-        self.client = create_box_runtime_client(ap, runtime_url=self.runtime_url)
+        self.client = RemoteBoxRuntimeClient(base_url=self.runtime_url, logger=ap.logger)
         self.runtime_subprocess: asyncio.subprocess.Process | None = None
         self.runtime_subprocess_task: asyncio.Task | None = None
 
@@ -54,9 +55,7 @@ class BoxRuntimeConnector:
             self.runtime_subprocess_task = None
 
     def _load_configured_runtime_url(self) -> str:
-        box_config = getattr(self.ap, 'instance_config', None)
-        box_config_data = getattr(box_config, 'data', {}) if box_config is not None else {}
-        return str(box_config_data.get('box', {}).get('runtime_url', '')).strip()
+        return str(get_box_config(self.ap).get('runtime_url', '')).strip()
 
     def _should_manage_local_runtime(self) -> bool:
         return not self.configured_runtime_url and platform.get_platform() != 'docker'