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/lib/realtime.ts

@@ -1,7 +1,7 @@
 import { canonicalizeWindowsPath, windowsPathKey } from './pathing';
 import type { ActivityEvent } from './activity';
 
-export type IssuesChangeKind = 'changed' | 'renamed';
+export type IssuesChangeKind = 'changed' | 'renamed' | 'telemetry';
 
 export interface IssuesChangedEvent {
   id: number;
@@ -184,7 +184,8 @@ if (!globalRegistry.__beadboardActivityEventBus) {
 }
 
 export function toSseFrame(event: IssuesChangedEvent): string {
-  return `id: ${event.id}\nevent: issues\ndata: ${JSON.stringify(event)}\n\n`;
+  const eventName = event.kind === 'telemetry' ? 'telemetry' : 'issues';
+  return `id: ${event.id}\nevent: ${eventName}\ndata: ${JSON.stringify(event)}\n\n`;
 }
 
 export function toActivitySseFrame(event: ActivityDispatchedEvent): string {