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_explorer_universal.py

@@ -20,10 +20,8 @@ from unittest.mock import AsyncMock, MagicMock, Mock, patch
 import pytest
 from langchain_core.messages import AIMessage
 
-from artemis.agents.explorer.explorer import (
-    UNIVERSAL_EXPLORER_TOOLS,
-    Explorer,
-)
+from artemis.agents.explorer.explorer import Explorer
+from artemis.agents.explorer.tool_declarations import UNIVERSAL_EXPLORER_TOOLS
 from artemis.config import settings
 from artemis.context import ArtemisContext
 from artemis.graph.state import State
@@ -106,7 +104,7 @@ async def test_run_universal_explorer_success(mock_context, tmp_path):
     mock_llm.bind_tools.return_value = mock_bound
 
     with patch(
-        "artemis.agents.explorer.explorer.get_llm",
+        "artemis.agents.explorer.universal_runner.get_llm",
         return_value=mock_llm,
     ):
         mock_state = Mock(spec=State)