Add --summary and --summary-json flags for statistics output

[nikfot]

Summary

• Adds --summary flag to visualize command that prints per-metric statistics (min, max, mean, p95, p99) as a formatted text table
• Adds --summary-json flag that outputs the same data as machine-readable JSON (skips plot generation)
• Includes alert thresholds: high CPU (user+sys > 80%), IO wait (> 20%), swap in use

Example output

============================================================
  vmstat Summary (30 samples)
============================================================

  CPU
  Metric                        Min     Max    Mean     P95     P99
  -------------------------------------------------------
  user_cpu_percent                0      15    3.83     7.0   12.68
  system_cpu_percent              0       5    1.93     3.0    4.42
  ...

Test plan

• Verified text output against example.log
• Verified JSON output is valid and parseable
• --summary-json suppresses plot generation