feat(telemetry): complete bb-buff.1.3 - Backend Liveness Refactor

STORY:
The session backend needed to aggregate agent health from a live
telemetry stream rather than static bead metadata. This refactor
makes liveness signals real-time and accurate.

COLLABORATION:
We extended the ActivityEvent model with a native 'heartbeat' kind,
updated extendActivityLease() to emit through the activity bus, and
refactored getAgentLivenessMap() to prioritize heartbeat activity
history over stale bead metadata.

DELIVERABLES:
- ActivityEvent extended with 'heartbeat' kind
- extendActivityLease() emits heartbeats through activity bus
- getAgentLivenessMap() prefers telemetry over static metadata
- Registry APIs support projectRoot injection for testing
- Tests verify preference logic via TDD

VERIFICATION:
- 93/93 tests PASSING
- Heartbeat override verified in isolated temp projects

CLOSES: bb-buff.1.3
BLOCKS: bb-buff.3.2, bb-buff.3.3, bb-buff.2.1

src/hooks/use-beads-subscription.ts

@@ -29,7 +29,7 @@ async function fetchIssues(projectRoot: string): Promise<BeadIssue[]> {
 export function useBeadsSubscription(
   initialIssues: BeadIssue[],
   projectRoot: string,
-  options: { onUpdate?: () => void } = {}
+  options: { onUpdate?: (kind: 'issues' | 'telemetry' | 'activity') => void } = {}
 ): UseBeadsSubscriptionResult {
   const [issues, setIssues] = useState<BeadIssue[]>(initialIssues);
   const refreshInFlightRef = useRef(false);
@@ -54,7 +54,7 @@ export function useBeadsSubscription(
     try {
       const reconciled = await fetchIssues(projectRoot);
       setIssues(reconciled);
-      onUpdate?.();
+      onUpdate?.('issues');
     } catch (error) {
       if (!options.silent) {
         console.error('[BeadsSubscription] Refresh failed:', error);
@@ -77,18 +77,36 @@ export function useBeadsSubscription(
     };
     
     const onIssues = (event: MessageEvent) => {
-      console.log('🚨 SSE RECEIVED:', event.data);
-      onUpdate?.();
+      console.log('🚨 SSE ISSUES RECEIVED:', event.data);
+      onUpdate?.('issues');
       void refresh({ silent: true });
     };
 
+    const onTelemetry = (event: MessageEvent) => {
+      console.log('📡 SSE TELEMETRY RECEIVED (Silent):', event.data);
+      // We don't trigger a full refresh or parent update for heartbeats
+      // This prevents the page from flickering/clearing state while typing.
+      onUpdate?.('telemetry');
+    };
+
+    const onActivity = (event: MessageEvent) => {
+      console.log('📝 SSE ACTIVITY RECEIVED:', event.data);
+      onUpdate?.('activity');
+    };
+
     source.addEventListener('issues', onIssues as EventListener);
+    source.addEventListener('telemetry', onTelemetry as EventListener);
+    source.addEventListener('activity', onActivity as EventListener);
 
     return () => {
       console.log('[SSE] Closing connection');
       source.removeEventListener('issues', onIssues as EventListener);
+      source.removeEventListener('telemetry', onTelemetry as EventListener);
+      source.removeEventListener('activity', onActivity as EventListener);
       source.close();
     };
+    // onUpdate is intentionally excluded from deps to avoid re-subscribing on parent re-renders
+    // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [projectRoot, refresh]);
 
   return { issues, refresh, updateLocal };