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

tests/api/events-route.test.ts

@@ -4,6 +4,10 @@ import assert from 'node:assert/strict';
 import { GET as eventsGet } from '../../src/app/api/events/route';
 import { getIssuesWatchManager } from '../../src/lib/watcher';
 
+test.afterEach(async () => {
+  await getIssuesWatchManager().stopAll();
+});
+
 test.after(async () => {
   await getIssuesWatchManager().stopAll();
 });