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/agents/test_operator_prompts.py

@@ -12,11 +12,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from artemis.agents.operator.prompt_builder import (
-    OperatorPromptBuilder,
+from artemis.agents.operator.prompts import (
+    apply_operator_prompt_contract,
     load_operator_prompts,
 )
-from artemis.agents.operator.prompts import apply_operator_prompt_contract
 
 
 def test_operator_prompt_contract_matches_note_runtime_semantics():
@@ -68,9 +67,8 @@ def test_operator_prompt_keeps_large_list_traversal_single_pass_until_boundary()
         assert "Terminate exploration when a definitive boundary is reached" not in prompt
 
 
-def test_compatibility_builder_uses_canonical_operator_template():
+def test_contract_strips_legacy_tool_literals():
     expected = apply_operator_prompt_contract(load_operator_prompts()["main_template"])
 
-    assert OperatorPromptBuilder.build_system_message() == expected
     assert "wait_for_delay(seconds=" not in expected
     assert 'press_key(key="home"' not in expected