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/utils/test_optional_ocr.py

@@ -21,7 +21,6 @@ from artemis.agents.explorer.explorer import Explorer
 from artemis.config.settings import Settings
 from artemis.graph.perception import perception_node
 from artemis.graph.state import State
-from artemis.tools.base import ToolRegistry
 from artemis.tools.index import get_tools_from_wrappers
 from artemis.tools.mobile.ocr import (
     ocr_recognition,
@@ -126,20 +125,8 @@ def test_ocr_tool_not_exposed_when_unconfigured():
     with patch("artemis.tools.mobile.ocr.is_ocr_configured", return_value=False):
         assert not ocr_recognition.is_available()
 
-        # ToolRegistry.list_tools filters out unavailable tools
-        available_tools = ToolRegistry.list_tools(available_only=True)
-        assert "ocr_recognition" not in [t.name for t in available_tools]
-
-        # ToolRegistry.get_langchain_tools filters out unavailable tools
-        mock_ctx = MagicMock()
-        lc_tools = ToolRegistry.get_langchain_tools(mock_ctx, available_only=True)
-        assert "ocr_recognition" not in [t.name for t in lc_tools]
-
-        # ToolRegistry.get_genai_declarations filters out unavailable tools
-        genai_tools = ToolRegistry.get_genai_declarations(available_only=True)
-        assert "ocr_recognition" not in [t.name for t in genai_tools]
-
         # get_tools_from_wrappers excludes ocr_recognition_wrapper
+        mock_ctx = MagicMock()
         wrapped_tools = get_tools_from_wrappers(mock_ctx, [ocr_recognition_wrapper])
         assert len(wrapped_tools) == 0