Arm monitor node

[allenabraham106]

• Added ArmMonitorNode as a composable node (rclcpp_components)
• Subscribes to /joint_states and republishes arm joint state data
• Monitors joints against configurable upper/lower limits and publishes /near_joint_limit warning
• Publishes /distance_to_box on a configurable timer (default 100ms)
• Subscribes to /is_executing to track arm execution state
• All joint limits, box position, and topic names configurable via ROS2 params — not hardcoded to panda