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/constants.py

@@ -22,6 +22,9 @@ Explorer tier the user configured.  Tier behavior lives in
 
 ASK_EXPLORER_TOOL_NAME = "ask_explorer"
 
+#: Model used when the LLM configuration carries no Explorer entry.
+DEFAULT_EXPLORER_MODEL = "gemini-3.8-flash"
+
 ASK_EXPLORER_DESCRIPTION = (
     "[EXPLORER] Ask the UI Explorer to locate elements on the current screen"
     " that are missing from the indexed element list or whose listed"