refactor(box): move box runtime to langbot-plugin-sdk

Extract self-contained box runtime modules (actions, backend, client,
  errors, models, runtime, security, server) to langbot-plugin-sdk and
  update all imports to use `langbot_plugin.box.*`. Keep only service
  and
  connector in LangBot core as they depend on the Application context.

  - Update docker-compose to use `langbot_plugin.box.server` entry
  point
  - Update pyproject.toml to use local SDK via `tool.uv.sources`
  - Remove migrated source files and their unit/integration tests
  - Update remaining test imports to match new module paths

src/langbot/pkg/provider/tools/loaders/mcp.py

@@ -327,7 +327,7 @@ class RuntimeMCPSession:
 
     async def _monitor_box_process_health(self):
         """Poll managed process status; return when process exits."""
-        from langbot.pkg.box.models import BoxManagedProcessStatus
+        from langbot_plugin.box.models import BoxManagedProcessStatus
 
         session_id = self._build_box_session_id()
         consecutive_errors = 0

src/langbot/pkg/provider/tools/loaders/native.py

@@ -5,7 +5,7 @@ import json
 import langbot_plugin.api.entities.builtin.resource.tool as resource_tool
 from langbot_plugin.api.entities.events import pipeline_query
 
-from langbot.pkg.box.models import BoxNetworkMode
+from langbot_plugin.box.models import BoxNetworkMode
 from .. import loader
 
 SANDBOX_EXEC_TOOL_NAME = 'sandbox_exec'