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/read-issues.ts

@@ -14,6 +14,7 @@ export interface ReadIssuesOptions {
   projectSource?: ProjectSource;
   projectAddedAt?: string | null;
   preferBd?: boolean;
+  skipAgentFilter?: boolean;
 }
 
 export function resolveIssuesJsonlPathCandidates(projectRoot: string = process.cwd()): string[] {
@@ -108,8 +109,8 @@ async function readIssuesViaBd(options: ReadIssuesOptions, project: ReturnType<t
       .filter((issue) => {
         // Exclude tombstones
         if (issue.status === 'tombstone' && !options.includeTombstones) return false;
-        // Exclude agent identities from mission lists
-        if (issue.labels.includes('gt:agent')) return false;
+        // Exclude agent identities from mission lists unless skipping filter (for watcher/diffing)
+        if (issue.labels.includes('gt:agent') && !options.skipAgentFilter) return false;
         return true;
       })
       .map((issue) => ({
@@ -141,6 +142,7 @@ export async function readIssuesFromDisk(options: ReadIssuesOptions = {}): Promi
       const jsonl = await readTextFileWithRetry(issuesPath);
       return parseIssuesJsonl(jsonl, {
         includeTombstones: options.includeTombstones ?? false,
+        skipAgentFilter: options.skipAgentFilter ?? false,
       }).map((issue) => ({
         ...issue,
         project,