Rename k8s's key from the custom k3s_homelab to the default
id_ed25519 - the custom name had no real justification (k8s is a
fresh account with nothing to collide with) and caused a real bug:
OpenSSH only auto-offers default-named keys, so ssh commands lacking
an explicit -i silently failed to authenticate. Also add the
never-actually-documented kubectl install step, and mkdir -p ~/.kube
before the first redirect into it, in both SETUP.md and QUICKSTART.md.
