Manage sllidar_ros2 as a git subtree and apply QoS fix

- Add m5p3nc3r/sllidar_ros2 fork as a git subtree at lidar/sllidar_ros2/
- Dockerfile now COPYs source from the workspace instead of cloning at
  build time; git is no longer a build dependency
- Set /scan publisher QoS to BEST_EFFORT (depth 2) for Nav2/RViz2 compat
- Add lidar/README.md documenting the subtree workflow
- Remove lidar/sllidar_ros2/ from .gitignore (no longer needed)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lidar/sllidar_ros2/src/sllidar_node.cpp

@@ -59,7 +59,7 @@ class SLlidarNode : public rclcpp::Node
     : Node("sllidar_node")
     {
 
-      scan_pub = this->create_publisher<sensor_msgs::msg::LaserScan>("scan", rclcpp::QoS(rclcpp::KeepLast(10)));
+      scan_pub = this->create_publisher<sensor_msgs::msg::LaserScan>("scan", rclcpp::QoS(rclcpp::KeepLast(2)).best_effort());
       
     }