Hold Climber Position with Motor Stalls

Here's my thought:

`GamepadClimberControl`: when the joystick input is between 20% and -20%, we engage the brake and set the motors to a known stall power (e.g. 15%).

Has the downside of possibly triggering the pneumatics a lot if we're holding the stick right around 20% (we could disengage at 30% and engage at 20% to prevent that).

Start by locating and reading GamepadClimberControl, then inspect how joystick thresholds, braking, motor power, and pneumatics are currently handled. The intended behavior is to hold the climber near neutral with brake engagement and a known stall power, while avoiding repeated pneumatic triggers around the threshold; done means this behavior is implemented and verified in operation.

Written by the indexing model from the issue text.