ax(node): replace prose comment with usage example on GetConnection

AX Principle 2 — comments must show HOW with real values, not restate
what the signature already says. The old comment described the return
value in prose; replaced with a concrete call-site example.

Co-Authored-By: Charon <charon@lethean.io>

pkg/node/transport.go

@@ -404,7 +404,8 @@ func (t *Transport) Broadcast(msg *Message) error {
 	return lastErr
 }
 
-// GetConnection returns an active connection to a peer.
+// conn := transport.GetConnection(peer.ID)
+// if conn == nil { return fmt.Errorf("peer not connected") }
 func (t *Transport) GetConnection(peerID string) *PeerConnection {
 	t.mutex.RLock()
 	defer t.mutex.RUnlock()