fix: require current a2a init scaffold

Dockerfile

@@ -4,6 +4,7 @@ WORKDIR /app
 
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
+RUN python -c "import importlib.resources as r, a2a_pack; t = r.files('a2a_pack.cli.templates').joinpath('agent.py.tmpl').read_text(); missing = [m for m in ('LLMProvisioning.CALLER_PROVIDED', 'ctx.llm', 'create_deep_agent', 'wrap_model_call') if m not in t]; assert not missing, f'a2a-pack {a2a_pack.__version__} has stale init template; missing {missing}'"
 
 COPY . .