Use qwen3.8-27b as the example executor

It led the executor benchmark (PR #55): 37/40 over eight tasks and 17/20
on the harder four, ahead of claude-haiku-4-5's 15/20. local-fast, the
previous example, scored 0/8 on the harder four. Model selection is on
the dev executor alone for now; the shared-host question (spec Part H)
stays open.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

nisse-dev/tests/test_config.py

@@ -272,7 +272,9 @@ def test_the_example_file_is_valid() -> None:
         "NISSE_MATRIX_TOKEN": "syt_x",
         "NISSE_GITEA_TOKEN": "gt",
     }
-    assert AgentConfig.from_file(example, env).tuning.executor.max_turns == 40
+    config = AgentConfig.from_file(example, env)
+    assert config.tuning.executor.max_turns == 40
+    assert config.roles.executor == "qwen3.8-27b"
 
 
 # --- nothing that carries secrets may sit inside the clone -----------------