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/tools/explorer_tool.py

@@ -56,7 +56,7 @@ from artemis.config import ExplorerVersion, resolve_explorer_version, settings
 from artemis.context import ArtemisContext
 from artemis.drivers.base import BaseDeviceDriver
 from artemis.graph.state import State
-from artemis.tools.base import ArtemisTool, ToolCategory, ToolRegistry
+from artemis.tools.base import ArtemisTool, ToolCategory
 from artemis.tools.tool_wrapper import ToolWrapper
 from artemis.tools.types import CyFunctionDetector
 from artemis.utils.element_hit_test import find_element_at_point
@@ -527,7 +527,6 @@ class AskExplorerTool(ArtemisTool):
 
 
 ask_explorer = AskExplorerTool()
-ToolRegistry.register(ask_explorer)
 
 
 def get_ask_explorer_tool(