artifact: apps/server/src/main/kotlin/com/correx/apps/server/protocol/ServerMessage.kt
purpose: Add RouterResponseMessage data class (sessionId: SessionId, content: String, steeringEmitted: Boolean) as the egress protocol type for router responses.
depends_on: task-008
non_goals: Implementing WebSocket handler logic, adding client message types
done_when: RouterResponseMessage serializes and deserializes through ProtocolSerializer without errors