l8k: Cluster Discovery

PREREQUISITE: Read ../k8s-launch-kit-shared/SKILL.md for install paths, global flags, and output modes.

This workflow uses the default host target; --target host is equivalent. The CLI snapshots discovery arguments and runs the concrete Host discovery operation through the target registry; command syntax and artifacts are unchanged.

Discover cluster hardware and produce a cluster-config.yaml describing NICs, GPUs, rails, and node groups. Use docs/user/profiles.md#check-prerequisites-and-site-inputs for the site-owned decisions and docs/user/discovery.md#review-saved-inventory for the post-discovery review before generation.