Add Build Remote Agent phone pairing (gbr/1)

[LinespottingPrivate]

Re-opened after inactivity close of #3856. Docs still current (gbr-agent v0.6+, protocol gbr/1). Independent product by LineSpotting AB; not affiliated with xAI/SpaceX.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}