feat: add Gemini Deep Research client (Task 3)

New service at lib/services/gemini-research.ts that uses the
@google/genai Interactions API for autonomous web research.

Exports:
- submitResearch(topic, config?) → interactionId
- pollResearch(interactionId) → ResearchStatus
- parseResearchReport(topic, report) → ResearchPayload
- conductGeminiResearch(topic, config?) → ResearchPayload (full pipeline)

All models and prompts configurable via Sanity pipelineConfig.
Output matches existing ResearchPayload interface for backward compat.

Co-authored-by: research <research@miriad.systems>

Author: Miriad