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.

tests/unit/tools/test_read_hierarchy_tool.py

@@ -16,7 +16,7 @@ from unittest.mock import AsyncMock, MagicMock
 
 from artemis.context import ArtemisContext
 from artemis.drivers.base import BaseDeviceDriver
-from artemis.tools.base import ArtemisTool, ToolRegistry
+from artemis.tools.base import ArtemisTool
 from artemis.tools.mobile.read_hierarchy import (
     GetUIHierarchy,
     GetUiHierarchy,
@@ -46,8 +46,8 @@ def mock_driver():
     return driver
 
 
-def test_get_ui_hierarchy_tool_subclass_and_registry():
-    """Verify GetUiHierarchyTool is a subclass of ArtemisTool and properly registered."""
+def test_get_ui_hierarchy_tool_subclass():
+    """Verify GetUiHierarchyTool is a subclass of ArtemisTool."""
     assert issubclass(GetUiHierarchyTool, ArtemisTool)
     assert issubclass(GetUiHierarchy, ArtemisTool)
     assert issubclass(GetUIHierarchy, ArtemisTool)
@@ -58,11 +58,6 @@ def test_get_ui_hierarchy_tool_subclass_and_registry():
     assert get_ui_hierarchy.category == "diagnostic"
     assert get_ui_hierarchy.args_schema == GetUiHierarchyArgs
 
-    # Registry lookup
-    reg_tool = ToolRegistry.get("get_ui_hierarchy")
-    assert reg_tool is not None
-    assert isinstance(reg_tool, GetUiHierarchyTool)
-
     # GenAI FunctionDeclaration export
     declaration = get_ui_hierarchy.to_genai_declaration()
     assert declaration.name == "get_ui_hierarchy"