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/components/shared/status-utils.tsx

@@ -50,6 +50,9 @@ export function sessionStateGlow(state: string): string {
   switch (state) {
     case 'active': return 'shadow-[0_0_12px_rgba(74,222,128,0.3)] border-emerald-500/30';
     case 'needs_input': return 'shadow-[0_0_12px_rgba(248,113,113,0.3)] border-rose-500/30';
+    case 'stuck': return 'ring-2 ring-red-500 animate-pulse shadow-[0_0_16px_rgba(239,68,68,0.5)]';
+    case 'dead': return 'opacity-40 grayscale';
+    case 'evicted': return 'opacity-60 grayscale-[0.5]';
     case 'stale': return 'opacity-60 grayscale-[0.5]';
     case 'completed': return 'opacity-80';
     default: return '';