Add weekly Friday cronjob to CI workflow

[Copilot]

This PR adds a scheduled cronjob to the GitHub Actions CI workflow to automatically run the full CI pipeline every Friday at midnight UTC.

Changes Made

• Added a schedule trigger to .github/workflows/CI.yaml with cron expression 0 0 * * 5
• Added descriptive comment explaining the schedule timing

Why This Change

Running CI on a weekly schedule helps:

• Catch dependency drift and environmental changes that might break the build
• Ensure the project remains buildable even during periods of low development activity
• Provide early warning of upstream dependency issues
• Maintain code quality through regular automated testing

Technical Details

The cron expression 0 0 * * 5 follows GitHub Actions standard format:

• 0 - minute 0 (top of the hour)
• 0 - hour 0 (midnight UTC)
• * - any day of month
• * - any month
• 5 - Friday (0=Sunday through 6=Saturday)

The scheduled runs will execute the same comprehensive CI checks as pull requests and pushes to main:

• Pre-commit hooks validation
• Nix package building
• Nix flake checks across all systems
• Full integration test suite

Backward Compatibility

This change is completely backward compatible. Existing triggers (pull requests and pushes to main branch) remain unchanged and continue to work exactly as before.

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