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/agent-reservations.ts

@@ -407,7 +407,8 @@ export async function releaseAgentReservation(
   try {
     const now = deps.now ? deps.now() : new Date().toISOString();
     const reservations = await readActiveReservations();
-    const existing = reservations.find((reservation) => reservation.scope === scope);
+    const normalizedScope = normalizePath(scope);
+    const existing = reservations.find((reservation) => reservation.scope === normalizedScope);
 
     if (!existing || isExpired(existing, now)) {
       if (existing && isExpired(existing, now)) {