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/app/api/events/route.ts

@@ -86,7 +86,7 @@ export async function GET(request: Request): Promise<Response> {
         }
         if (nextVersion !== lastTouchedVersion) {
           lastTouchedVersion = nextVersion;
-          write(toSseFrame(issuesEventBus.emit(projectRoot, lastTouchedPath, 'changed')));
+          write(toSseFrame(issuesEventBus.emit(projectRoot, lastTouchedPath, 'telemetry')));
         }
       };
 

src/app/api/sessions/route.ts

@@ -14,7 +14,7 @@ export async function GET(request: Request): Promise<Response> {
     const issues = await readIssuesFromDisk({ projectRoot, preferBd: true });
     const activity = activityEventBus.getHistory(projectRoot);
     const communication = await getCommunicationSummary();
-    const livenessMap = await getAgentLivenessMap();
+    const livenessMap = await getAgentLivenessMap(projectRoot, activity);
     const incursions = await calculateIncursions();
     const agentsResult = await listAgents({});