Add Policy Evaluation Heatmap Widget to SRE Dashboard

Package: packages/agent-sre/

What

Add a heatmap visualization showing policy evaluation density over time — which agents trigger the most evaluations and when violations cluster.

Why

Operators need to spot patterns like "Agent X hits content policy limits every Tuesday at 3pm." A heatmap makes this visually obvious.

How

1. Look at packages/agent-sre/examples/dashboard/
2. Add a heatmap widget (agent x time or policy x time)
3. Data source: governance metrics via OpenTelemetry
4. Use existing charting library in the project

Acceptance Criteria

• Heatmap renders in the SRE dashboard
• X-axis: time buckets; Y-axis: agent IDs or policy names
• Color intensity: evaluation/violation count
• Tooltip on hover shows exact count