feat: add CPU usage to agent response and compact cards

Backend:
- Add cpuUsage field to AgentInstanceResponse (-1 if unavailable)
- Add queryAgentCpuUsage() to AgentRegistrationController — queries
  avg CPU per instance from agent_metrics over last 2 minutes
- Wire CPU into agent list response via withCpuUsage()

Frontend:
- Add cpuUsage to schema.d.ts
- Compute maxCpu per AppGroup (max across all instances)
- Show "X% cpu" on compact cards when available (hidden when -1)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/AgentHealth/AgentHealth.module.css

@@ -321,6 +321,11 @@
   color: var(--text-muted);
 }
 
+.compactCardCpu {
+  font-family: var(--font-mono);
+  color: var(--text-muted);
+}
+
 .compactCardHeartbeat {
   color: var(--text-muted);
 }