Generate a standardized empirical research brief from an idea, research question, theory/mechanism hints, design options, data hints, and optional web-grounding.

Use this before running econometric models when the user is still shaping the topic, identification strategy, or data plan.

Outputs:
- `research_brief.md`
- `research_brief.json`
- `research_brief_sources.json`

Rules:
- Separate the recommended design from alternative designs.
- State identifying assumptions, minimum data requirements, feasibility risks, and validity risks explicitly.
- Web grounding may only support public facts, policy background, and literature-style context. It must not invent causal identification.
- If web grounding fails, still produce the brief and mark `grounding_mode=offline_fallback`.
