Move runtime check rule zips to external repo (gim-home/ModelKitArtifacts)

[timenick]

Problem

The src/winml/modelkit/analyze/rules/runtime_check_rules/ directory contains 20 zip files (~312 MB) tracked by git, bloating clone size and git history.

Solution

PR #213 moves these zips to gim-home/ModelKitArtifacts with:

• .gitignore to exclude zips from git tracking
• scripts/download_rules.py for sparse checkout download from source repo
• runtime_check_rules/README.md with setup instructions
• Improved warning messages when zips are missing

See #213 for full details.