Hydrate screenshot proof for sandboxed reviewers

ClawSweeper allowed screenshots as real-behavior proof, but the media hydration path only collected video URLs. Read-only review sandboxes run without network access, so image-only proof had no local artifact and could be marked insufficient regardless of quality.

Generalized video proof discovery into media proof discovery, added bounded hydration for PNG, JPEG, WebP, and GIF image proof, branched artifact preparation by image versus video, and updated runtime prompt guidance to inspect local image paths before browser fallback.

• Lets screenshot-only evidence be judged on its merits inside network-disabled review sandboxes.
• Preserves existing video ffprobe/contact-sheet processing while skipping unnecessary video tooling for images.
• Keeps local-range review behavior safe by continuing to avoid host-side media downloads for synthetic local review bodies.

• Added regression coverage for image discovery, image artifact preparation, mixed image/video ordering, and preserved video-only hooks.
• Validated the production helper path with a real local HTTP-served PNG and bounded curl download.
• Full local check passed with build, lint, 1,180 unit tests, repair tests, changed and full coverage suites, and formatting.