GO2 - document `dimos go2tool` in go2 getting started

[leshy]

dimos go2tool (added in #1990, dimos/robot/unitree/go2/cli/go2tool.py) covers two setup steps that aren't in docs/platforms/quadruped/go2/index.md:

• dimos go2tool discover — scan BLE + LAN, prints SERIAL / NAME / IP / MAC. answers "what's ROBOT_IP?"
• dimos go2tool connect-wifi --serial <SN> --ssid <...> --password <...> — provision wifi over BLE without the unitree app

right now the docs jump straight to export ROBOT_IP=<YOUR_GO2_IP> with no hint how to get there. should be a new "First-time setup" subsection above "Pre-flight checks" that walks through:

1. power on the robot (BLE advertises immediately)
2. dimos go2tool connect-wifi --name Go2_XXXXX --ssid <wifi> --password <pw> — only needed once per network
3. dimos go2tool discover --lan — grab the IP, export it

depends on #1990 landing in dev first.

[leshy]

actually adressed in the original PR #1990