Public: Mastra cookbook RAG reference

Architectural notes

Mastra's public docs and cookbook ship a canonical RAG-over-docs
example that exercises the framework's retrieval, agent, and eval
primitives end-to-end.

Notable architectural choices (from the public cookbook):
- Provider-agnostic LLM and embedding layer.
- Built-in eval primitives co-located with agent definitions.
- Vercel-friendly deployment as the default target.
- Workflow primitives for multi-step pipelines.

Useful as a public reference for TypeScript-native RAG stacks built
with batteries-included frameworks.