review: capture kill channel once before the keep-alive loop

Reading getKillChannel(userID) on every loop iteration re-locked killchannelMu
once per second per session (contention under load) and could switch to a
replaced channel after a reconnect. Capture the channel once before the loop so
each session goroutine listens on its own channel without re-locking.
Addresses Gemini Code Assist review on #318.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Thiago Bauken

wmiau.go

@@ -653,10 +653,11 @@ func (s *server) startClient(userID string, textjid string, token string, subscr
 	}
 
 	// Keep connected client live until disconnected/killed. Read the kill
-	// channel through the mutex-guarded helper each iteration so this read
-	// never races with concurrent map writes/deletes from request goroutines.
+	// channel once through the mutex-guarded helper; the goroutine then listens
+	// on its own channel for the rest of its life, so it neither re-locks the
+	// mutex every second nor risks switching to a replaced channel.
+	kill, _ := getKillChannel(userID)
 	for {
-		kill, _ := getKillChannel(userID)
 		select {
 		case <-kill:
 			log.Info().Str("userid", userID).Msg("Received kill signal")