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({});
 

src/app/sessions/page.tsx

@@ -1,8 +1,10 @@
 import { SessionsPage } from '../../components/sessions/sessions-page';
+import type { SwarmGroup } from '../../components/sessions/sessions-header';
 import { readIssuesForScope } from '../../lib/aggregate-read';
 import { resolveProjectScope } from '../../lib/project-scope';
 import { listProjects } from '../../lib/registry';
 import { listAgents } from '../../lib/agent-registry';
+import { getSwarmMembers } from '../../lib/swarm-molecules';
 
 export const dynamic = 'force-dynamic';
 
@@ -32,6 +34,35 @@ export default async function Page({ searchParams }: PageProps) {
     preferBd: true,
   });
 
+  const epics = issues.filter(i => i.issue_type === 'epic');
+  const epicsWithSwarm = epics.filter(
+    i => (i.labels || []).some(l => l.startsWith('swarm:'))
+  );
+
+  const swarmGroups: SwarmGroup[] = [];
+  const assignedAgentIds = new Set<string>();
+
+  for (const epic of epicsWithSwarm) {
+    const swarmLabel = epic.labels?.find(l => l.startsWith('swarm:'));
+    if (!swarmLabel) continue;
+    
+    const swarmId = swarmLabel.replace('swarm:', '');
+    const memberIds = await getSwarmMembers({ swarmId }, { projectRoot: scope.selected.root });
+
+    const members = agents.filter(a => memberIds.includes(a.agent_id));
+    members.forEach(a => assignedAgentIds.add(a.agent_id));
+
+    if (members.length > 0) {
+      swarmGroups.push({
+        swarmId,
+        swarmLabel: epic.id,
+        members,
+      });
+    }
+  }
+
+  const unassignedAgents = agents.filter(a => !assignedAgentIds.has(a.agent_id));
+
   return (
     <SessionsPage
       issues={issues}
@@ -40,6 +71,8 @@ export default async function Page({ searchParams }: PageProps) {
       projectScopeKey={scope.selected.key}
       projectScopeOptions={scope.options}
       projectScopeMode={scope.mode}
+      swarmGroups={swarmGroups}
+      unassignedAgents={unassignedAgents}
     />
   );
 }