fix(review): inspect video proof with ffmpeg

[Takhoffman]

Summary

• preprocess linked video proof with ffprobe/ffmpeg before Codex review
• write a media proof manifest plus contact sheet into the proof scratch directory
• prompt Codex to inspect prepared ffmpeg artifacts before browser playback fallback
• cover browser-unplayable but ffmpeg-decodeable video proof in unit tests

Validation

• pnpm run build
• node --test test/clawsweeper.test.ts
• pnpm exec oxfmt --check src/clawsweeper.ts test/clawsweeper.test.ts
• pnpm run check