feat: Add CloudWatch Metrics browser with graph rendering (M3.8)

[nathanhuh]

Summary

Add CloudWatch Metrics browsing with in-terminal graph rendering.

Details

• List namespaces → metrics → dimensions
• Select metric + time range + period (statistics: Average, Sum, Min, Max, p99, etc.)
• Graph rendering in Bubbletea viewport using braille/block characters
• Support multiple metrics overlay on a single chart
• Common presets for popular metrics (CPU, memory, request count, latency)

Checklist

• Add CloudWatchMetrics service and feature constants in internal/domain/model.go
• Register in catalog via internal/domain/catalog.go
• Implement ListMetrics(), GetMetricData() in internal/services/aws/cloudwatch_metrics.go
• Add cloudwatch SDK dependency in go.mod
• Add NamespaceList, MetricList, MetricGraph screens in internal/app/
• Implement braille/block character graph renderer
• Add time range and period selector components
• Support multiple metric overlay
• Write tests