feat(dashboard): add Agent Regression Tracker

Deployment regression detection dashboard with:
- Welch's t-test comparison across consecutive deployments
- Cohen's d effect size filtering for meaningful regressions
- Configurable significance level and effect size cutoffs
- Color-coded deployment timeline
- Canvas metric charts with sparkline trends
- Auto-monitor mode with real-time toast notifications
- Cross-metric root cause hypothesis generation
- Proactive recommendations for each regression

Author: sauravbhattacharya001