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/agents/test_operator_transcript.py

@@ -33,7 +33,7 @@ import pytest
 from langchain_core.messages import HumanMessage, SystemMessage
 
 from artemis.agents.operator.operator import OperatorNode
-from artemis.agents.operator.prompt_builder import load_operator_prompts
+from artemis.agents.operator.prompts import load_operator_prompts
 from artemis.agents.operator.prompts import (
     PLAN_HISTORY_STATIC_POINTER,
     PLAN_HISTORY_TEMPLATE_SECTION,