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_video_analyzer_pure_tool.py

@@ -15,7 +15,7 @@
 from unittest.mock import AsyncMock, MagicMock, patch
 
 from artemis.context import ArtemisContext
-from artemis.tools.base import ArtemisTool, ToolRegistry
+from artemis.tools.base import ArtemisTool
 from artemis.tools.video_tool import (
     VideoAnalyzerArgs,
     VideoAnalyzerPure,
@@ -32,8 +32,8 @@ def mock_ctx():
     return ctx
 
 
-def test_video_analyzer_pure_tool_subclass_and_registry():
-    """Verify VideoAnalyzerPureTool is a subclass of ArtemisTool and properly registered."""
+def test_video_analyzer_pure_tool_subclass():
+    """Verify VideoAnalyzerPureTool is a subclass of ArtemisTool."""
     assert issubclass(VideoAnalyzerPureTool, ArtemisTool)
     assert issubclass(VideoAnalyzerPure, ArtemisTool)
     assert isinstance(video_analyzer_pure, ArtemisTool)
@@ -43,11 +43,6 @@ def test_video_analyzer_pure_tool_subclass_and_registry():
     assert video_analyzer_pure.category == "perception"
     assert video_analyzer_pure.args_schema == VideoAnalyzerArgs
 
-    # Registry lookup
-    reg_tool = ToolRegistry.get("video_analyzer_pure")
-    assert reg_tool is not None
-    assert isinstance(reg_tool, VideoAnalyzerPureTool)
-
     # GenAI FunctionDeclaration export
     declaration = video_analyzer_pure.to_genai_declaration()
     assert declaration.name == "video_analyzer_pure"