Add actor_pid to session_start event payload (gt-3zsml)

Adds a stable actor-PID identifier alongside the Claude session_id.
Useful for correlation across session handoffs.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/events/events.go

@@ -283,6 +283,7 @@ func SessionPayload(sessionID, role, topic, cwd string) map[string]interface{} {
 	p := map[string]interface{}{
 		"session_id": sessionID,
 		"role":       role,
+		"actor_pid":  fmt.Sprintf("%s-%d", role, os.Getpid()),
 	}
 	if topic != "" {
 		p["topic"] = topic