fix: correct task scheduling and simplify agent internals

Fix queue cancellation, concurrency limits and worker status updates.

Use direct Explorer imports, split the Diagnoser loop into helpers and remove unused tool registration and prompt-builder code.

Correct model caching, endpoint selection and retry handling.

artemis/__init__.py

@@ -22,7 +22,6 @@ from artemis_client import Capabilities, Device, TaskHandle, TaskResult
 from artemis.sdk.agent import Agent
 from artemis.sdk.builders import Builders
 from artemis.drivers.base import BaseDeviceDriver
-from artemis.tools.base import artemis_tool, ToolRegistry
 
 __all__ = [
     "__version__",
@@ -38,6 +37,4 @@ __all__ = [
     "TaskHandle",
     "TaskResult",
     "BaseDeviceDriver",
-    "artemis_tool",
-    "ToolRegistry",
 ]