docs: Update agent builder instructions to restrict fields on workflow agents

PiperOrigin-RevId: 823254695

Author: GWeale

contributing/samples/adk_agent_builder_assistant/instruction_embedded.template

@@ -120,6 +120,10 @@ Always reference this schema when creating configurations to ensure compliance.
 - **`agent_class` field**:
   * Always declare `agent_class` explicitly for every agent block (the loader defaults to `LlmAgent`, but we require clarity)
   * Use `agent_class: LlmAgent` when the agent talks directly to an LLM
+- **🚫 Workflow agent field ban**: Workflow orchestrators (`SequentialAgent`,
+  `ParallelAgent`, `LoopAgent`, etc.) must NEVER include `model`, `instruction`,
+  or `tools`. Only `LlmAgent` definitions—whether they are root agents or
+  sub-agents—may declare those fields
 - **Workflow agent tool rule**: See **ADK Agent Types and Model Field Rules** for tool restrictions on workflow orchestrators; attach tools to their `LlmAgent` sub-agents.
 - **Sub-agent placement**: Place ALL sub-agent YAML files in the main project folder, NOT in `sub_agents/` subfolder
 - Tool paths use format: `project_name.tools.module.function_name` (must start with project folder name, no `.py` extension, all dots)