feat: replace Errors column with CPU in expanded agent table

Show per-instance CPU usage percentage instead of error rate in the
DataTable. Highlights >80% CPU in error color.

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

ui/src/pages/AgentHealth/AgentHealth.tsx

@@ -386,13 +386,16 @@ export default function AgentHealth() {
         ),
       },
       {
-        key: 'errorRate',
-        header: 'Errors',
-        render: (_val, row) => (
-          <MonoText size="xs" className={row.errorRate ? styles.instanceError : styles.instanceMeta}>
-            {formatErrorRate(row.errorRate)}
-          </MonoText>
-        ),
+        key: 'cpuUsage',
+        header: 'CPU',
+        render: (_val, row) => {
+          const cpu = (row as AgentInstance & { cpuUsage?: number }).cpuUsage;
+          return (
+            <MonoText size="xs" className={cpu != null && cpu > 0.8 ? styles.instanceError : styles.instanceMeta}>
+              {cpu != null && cpu >= 0 ? `${(cpu * 100).toFixed(0)}%` : '\u2014'}
+            </MonoText>
+          );
+        },
       },
       {
         key: 'lastHeartbeat',