Gemini 3 Flash in the Search Subagent

Amp's codebase search subagent now uses Gemini 3 Flash instead of Haiku 4.5. It's 3x faster for the same quality.

Gemini 3 Flash is better at parallel tool calls, explores with diverse queries, and concludes early. Where Haiku 4.5 averaged ~2.5 parallel calls per iteration, Gemini 3 Flash fires off ~8. It wraps up in ~3 turns instead of ~9.