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_explorer_tool.py

@@ -27,7 +27,7 @@ from artemis.agents.explorer.constants import (
 )
 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.explorer_tool import (
     AskExplorerArgs,
     AskExplorerTool,
@@ -122,9 +122,7 @@ def test_description_does_not_depend_on_the_tier(version):
     assert tool.version == version
 
 
-def test_registry_and_wrapper_export():
-    reg_tool = ToolRegistry.get("ask_explorer")
-    assert isinstance(reg_tool, AskExplorerTool)
+def test_wrapper_export():
     assert ask_explorer_wrapper.tool_fn_getter is get_ask_explorer_tool
     assert ask_explorer_wrapper.on_success_fn("x") == "x"
     assert "boom" in ask_explorer_wrapper.on_failure_fn("boom")