Implement schema validation for ReviewSnapshot artifacts

## Problem
The system currently relies solely on filename patterns to identify legitimate review snapshots. This creates a vulnerability where corrupted or misplaced files could compromise the tracking system.

## Solution
- Implement proper schema validation for ReviewSnapshot objects
- Validate artifact contents beyond just filename checking
- Add validation before processing downloaded artifacts

## Related
PR #2 review feedback

Locate the ReviewSnapshot definition and the code path that processes downloaded artifacts; then read the related PR #2 review feedback for existing constraints. Done means artifact contents are validated against a schema before processing, rather than relying only on filename patterns.

Written by the indexing model from the issue text.