Normalize processor utilization

[samuel-williams-shopify]

Summary

• restore Linux General#processor_utilization using cumulative processor and elapsed time
• normalize both Linux /proc and ps pcpu results to core units, where 1.0 represents one fully occupied CPU core
• document that utilization is not bounded to 0.0..1.0 and may exceed 1.0 for multi-threaded processes
• add deterministic coverage for conversion of ps pcpu percentages and regenerate the agent context

Testing

• bundle exec bake test
• bundle exec rubocop (36 files, no offenses)
• bundle exec bake decode:index:coverage lib (41/41 public definitions documented)
• git diff --check