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_save_note_tool.py

@@ -17,7 +17,7 @@ from unittest.mock import MagicMock, patch
 
 from artemis.context import ArtemisContext
 from artemis.graph.state import State
-from artemis.tools.base import ArtemisTool, ToolRegistry
+from artemis.tools.base import ArtemisTool
 from artemis.tools.scratchpad import (
     SaveNote,
     SaveNoteArgs,
@@ -39,8 +39,8 @@ def mock_ctx(tmp_path):
     return ctx
 
 
-def test_save_note_tool_subclass_and_registry():
-    """Verify SaveNoteTool is a subclass of ArtemisTool and properly registered in ToolRegistry."""
+def test_save_note_tool_subclass():
+    """Verify SaveNoteTool is a subclass of ArtemisTool."""
     assert issubclass(SaveNoteTool, ArtemisTool)
     assert issubclass(SaveNote, ArtemisTool)
     assert isinstance(save_note, ArtemisTool)
@@ -50,11 +50,6 @@ def test_save_note_tool_subclass_and_registry():
     assert save_note.category == "memory"
     assert save_note.args_schema == SaveNoteArgs
 
-    # Registry lookup
-    reg_tool = ToolRegistry.get("save_note")
-    assert reg_tool is not None
-    assert isinstance(reg_tool, SaveNoteTool)
-
     # GenAI FunctionDeclaration export
     declaration = save_note.to_genai_declaration()
     assert declaration.name == "save_note"