feat: Gemini Deep Research client (Task 3)

New Gemini Deep Research client using @google/genai Interactions API.\n\nExports: submitResearch, pollResearch, parseResearchReport, conductGeminiResearch.\nAll models/prompts from Sanity config. Two-phase parsing (Gemini Flash + regex fallback).\nBackward-compatible ResearchPayload output.

Author: codercatdev