Add task_24_polymarket_briefing: prediction markets + news cross-reference

[Aleschka]

Summary

Adds task_24_polymarket_briefing — fetches live Polymarket prediction markets and cross-references each with a recent news story.

Category: Research
Grading: Hybrid (automated + LLM judge)
Timeout: 180s

What it tests

1. Real-time API access — Polymarket's public gamma-api (no auth required)
2. Numerical data integrity — odds must be plausible, not hallucinated
3. News cross-referencing — finding news that explains why a market is active
4. Format compliance — structured markdown with labeled fields

Why it's useful for v2

This is one of the harder "real-time data + reasoning" tasks because:

• Models that can't fetch data will try to invent market odds (easily caught by LLM judge)
• Connecting market activity to news requires actual search, not recall
• Tests a practical agentic use case: prediction markets as news importance signals

API reference

GET https://gamma-api.polymarket.com/markets?active=true&order=volumeNum&ascending=false&limit=10

No authentication required.

Part of PinchBench v2 community task expansion (issue #60).