feat: replace fixture releases with real GitHub releases data

- fetch_github_releases() pulls real tagged releases from GitHub API
- Issues now filtered to last 3 months via since= param
- Falls back to latest 10 releases if none found in time window
- Dataset: 64 issues + 18 real releases (7 Flutter, 11 React Native)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Jaspreet Singh