feat(metrics): add disk usage tracking with test demonstration

[Copilot]

Adds disk usage as a third metric alongside CPU and memory. Test workflow now exercises disk allocation to validate the feature.

Metrics Collection

• Added diskUsageGBs to schema tracking used/free space in GB
• Collects via systeminformation.fsSize(), summing all mounted filesystems
• Integrated into both periodic collection (5s intervals) and final snapshot

Visualization

• Renders "Disk Usages" chart with Cyan (Free) / Purple (Used) stacked bars
• Y-axis in GB, consistent with other metrics using contextual workflow step labels

Test Coverage

• Unit tests validate collection, accumulation, and byte-to-GB conversion
• test-resource-visualization job now creates/deletes 5GB file during CPU step:

  dd if=/dev/zero of=/tmp/test-5gb-file.bin bs=1M count=5120
  # ... CPU intensive work ...
  rm -f /tmp/test-5gb-file.bin

• Demonstrates ~5GB spike and cleanup in disk usage chart

Implementation

Extracted bytesPerMB and bytesPerGB to src/lib.ts as shared constants (addresses code review feedback on duplication).

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