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.

artemis/agents/explorer/tool_declarations.py

@@ -17,8 +17,7 @@
 Split out of ``artemis.agents.explorer.explorer`` as pure data:
 ``UNIVERSAL_EXPLORER_TOOLS`` (OpenAI-style dict schemas for the universal
 LangChain path) and ``NATIVE_EXPLORER_TOOL_DECLARATIONS`` (google-genai
-``FunctionDeclaration`` objects for the native Gemini path).  Both are
-re-exported by the ``explorer`` facade module.
+``FunctionDeclaration`` objects for the native Gemini path).
 """
 
 from google.genai import types