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.example.yaml

@@ -23,7 +23,7 @@ llm:                                  # A5: LiteLLM is the only LLM destination
   request_timeout_s: 600
 
 roles:                                # A11: role -> LiteLLM model alias
-  executor: local-fast          # the local tier; the benchmark picks the model
+  executor: qwen3.8-27b         # the local tier: best executor in the benchmark (PR #55)
   planner: claude-opus-5-5      # high-end judgement: plans a weaker executor follows
   delegate: claude-opus-5-5     # takes over what the executor got stuck on
   consultant: claude-haiku-4-5  # one question at a time, no tools: cheap is enough