Docking on non-main branches silently fails because rig identity beads
live on main. The dock appeared to work but was lost on checkout to main.
Now dock/undock check current branch and error with helpful message:
"cannot dock: must be on main branch (currently on X)"
Fixes hq-kc7
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>