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_image_processor_tool.py

@@ -64,9 +64,9 @@ async def test_ask_vision_coder_success():
         assert result == vision_coder_outcome
 
 
-def test_ask_image_processor_tool_subclass_and_registry():
-    """Verify AskImageProcessorTool is an ArtemisTool subclass and registered in ToolRegistry."""
-    from artemis.tools.base import ArtemisTool, ToolRegistry
+def test_ask_image_processor_tool_subclass():
+    """Verify AskImageProcessorTool is an ArtemisTool subclass."""
+    from artemis.tools.base import ArtemisTool
     from artemis.tools.image_processor_tool import (
         AskImageProcessor,
         AskImageProcessorTool,
@@ -87,11 +87,6 @@ def test_ask_image_processor_tool_subclass_and_registry():
     assert ask_image_processor.category == "custom"
     assert ask_image_processor.args_schema == AskVisionCoderArgs
 
-    # Registry lookup
-    reg_tool = ToolRegistry.get("ask_image_processor")
-    assert reg_tool is not None
-    assert isinstance(reg_tool, AskImageProcessorTool)
-
     # GenAI FunctionDeclaration export
     declaration = ask_image_processor.to_genai_declaration()
     assert declaration.name == "ask_image_processor"