fix: show N/A for agent heap/CPU when no JVM metrics available

Indeterminate progress bars were misleading when agents don't report
JVM metrics — replaced with plain "N/A" text.

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

ui/src/pages/AgentHealth/AgentHealth.tsx

@@ -146,27 +146,33 @@ function AgentOverviewContent({ agent }: { agent: AgentInstance }) {
       </div>
       <div className={styles.detailRow}>
         <span className={styles.detailLabel}>Heap Memory</span>
-        <div className={styles.detailProgress}>
-          <ProgressBar
-            value={heapPercent}
-            variant={heapPercent == null ? 'primary' : heapPercent > 85 ? 'error' : heapPercent > 70 ? 'warning' : 'success'}
-            indeterminate={heapPercent == null}
-            size="sm"
-          />
-          <MonoText size="xs">{heapPercent != null ? `${heapPercent}%` : '\u2014'}</MonoText>
-        </div>
+        {heapPercent != null ? (
+          <div className={styles.detailProgress}>
+            <ProgressBar
+              value={heapPercent}
+              variant={heapPercent > 85 ? 'error' : heapPercent > 70 ? 'warning' : 'success'}
+              size="sm"
+            />
+            <MonoText size="xs">{heapPercent}%</MonoText>
+          </div>
+        ) : (
+          <MonoText size="xs">N/A</MonoText>
+        )}
       </div>
       <div className={styles.detailRow}>
         <span className={styles.detailLabel}>CPU</span>
-        <div className={styles.detailProgress}>
-          <ProgressBar
-            value={cpuPercent}
-            variant={cpuPercent == null ? 'primary' : cpuPercent > 80 ? 'error' : cpuPercent > 60 ? 'warning' : 'success'}
-            indeterminate={cpuPercent == null}
-            size="sm"
-          />
-          <MonoText size="xs">{cpuPercent != null ? `${cpuPercent}%` : '\u2014'}</MonoText>
-        </div>
+        {cpuPercent != null ? (
+          <div className={styles.detailProgress}>
+            <ProgressBar
+              value={cpuPercent}
+              variant={cpuPercent > 80 ? 'error' : cpuPercent > 60 ? 'warning' : 'success'}
+              size="sm"
+            />
+            <MonoText size="xs">{cpuPercent}%</MonoText>
+          </div>
+        ) : (
+          <MonoText size="xs">N/A</MonoText>
+        )}
       </div>
     </div>
   );