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/api/schema.d.ts

@@ -2065,6 +2065,8 @@ export interface components {
             totalRoutes: number;
             /** Format: int64 */
             uptimeSeconds: number;
+            /** Format: double */
+            cpuUsage: number;
         };
         SseEmitter: {
             /** Format: int64 */