Add optional metrics export (--metrics flag)

[rz1989s]

Summary

Add optional metrics export functionality for monitoring and analytics.

Background

From production readiness audit (2025-12-13): No metrics export capability for advanced monitoring use cases.

Requirements

• Add `--metrics` flag to statusline.sh
• Export performance metrics (load times, cache hits/misses)
• Support multiple output formats (JSON, Prometheus)
• Make feature opt-in to maintain simplicity
• Document metrics in cli-reference.md

Proposed Metrics

{
  "timestamp": 1702483200,
  "execution_time_ms": 45,
  "cache": {
    "hits": 8,
    "misses": 2,
    "hit_rate": 0.80
  },
  "modules": {
    "loaded": 11,
    "load_time_ms": 12
  },
  "components": {
    "rendered": 6,
    "render_time_ms": 28
  }
}

Prometheus Format

statusline_execution_time_ms 45
statusline_cache_hit_rate 0.80
statusline_modules_loaded 11

Use Cases

• Performance monitoring dashboards
• Debugging slow statusline renders
• Usage analytics

Priority

🟢 Low Priority - Advanced feature

References

• Production Readiness Report: docs/production-readiness-report.md