fix: remove 307 accidentally committed data files, update .gitignore

PR #227 accidentally committed local experiment data via git add -A:
- flywheel_results/ (224 screenshots + JSON)
- .claude/worktrees/ (31 agent gitlinks)
- annotated_demos/ (16 files)
- eval_results/ (11 screenshots)
- grpo_output/ (1 file)
- demos/*/synthetic_correction/ (placeholder PNGs)
- .beads/ (SQLite database)

All removed from tracking. .gitignore updated to prevent reoccurrence.
No sensitive data was exposed (confirmed via tidy scan).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: abrichr